array_keys

(PHP 4, PHP 5, PHP 7, PHP 8)

array_keys — Return all the keys or a subset of the keys of an array

Description

array_keys(array $array): array

array_keys(array $array, mixed $filter_value, bool $strict = false): array

array_keys() returns the keys, numeric and string, from the array.

If a filter_value is specified, then only the keys for that value are returned. Otherwise, all the keys from the array are returned.

Parameters

array: An array containing keys to return.
filter_value: If specified, then only keys containing this value are returned.
strict: Determines if strict comparison (===) should be used during the search.

Return Values

Returns an array of all the keys in array.

Examples

Example #1 array_keys() example

<?php
$array = array(0 => 100, "color" => "red");
print_r(array_keys($array));

$array = array("blue", "red", "green", "blue", "blue");
print_r(array_keys($array, "blue"));

$array = array("color" => array("blue", "red", "green"),
               "size"  => array("small", "medium", "large"));
print_r(array_keys($array));
?>

The above example will output:

Array
(
    [0] => 0
    [1] => color
)
Array
(
    [0] => 0
    [1] => 3
    [2] => 4
)
Array
(
    [0] => color
    [1] => size
)

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User Contributed Notes 27 notes

down

144

pat dot leblanc at gmail dot com ¶

13 years ago

It's worth noting that if you have keys that are long integer, such as '329462291595', they will be considered as such on a 64bits system, but will be of type string on a 32 bits system.

for example:
<?php 

$importantKeys = array('329462291595' =>null, 'ZZ291595' => null);

foreach(array_keys($importantKeys) as $key){
    echo gettype($key)."\n";
}

?>

will return on a 64 bits system:
<?php 
    integer
    string
?>

but on a 32 bits system:
<?php 
    string
    string
?>

I hope it will save someone the huge headache I had :)

down

Sven (bitcetera.com) ¶

19 years ago

Here's how to get the first key, the last key, the first value or the last value of a (hash) array without explicitly copying nor altering the original array:

<?php
  $array = array('first'=>'111', 'second'=>'222', 'third'=>'333');

  // get the first key: returns 'first'
  print array_shift(array_keys($array));

  // get the last key: returns 'third'
  print array_pop(array_keys($array));

  // get the first value: returns '111'
  print array_shift(array_values($array));

  // get the last value: returns '333'
  print array_pop(array_values($array));
?>

down

Ian (maxianos at hotmail dot com) ¶

10 years ago

There's a lot of multidimensional array_keys function out there, but each of them only merges all the keys in one flat array.

Here's a way to find all the keys from a multidimensional  array while keeping the array structure. An optional MAXIMUM DEPTH parameter can be set for testing purpose in case of very large arrays.

NOTE: If the sub element isn't an array, it will be ignore.

<?php
function array_keys_recursive($myArray, $MAXDEPTH = INF, $depth = 0, $arrayKeys = array()){
       if($depth < $MAXDEPTH){
            $depth++;
            $keys = array_keys($myArray);
            foreach($keys as $key){
                if(is_array($myArray[$key])){
                    $arrayKeys[$key] = array_keys_recursive($myArray[$key], $MAXDEPTH, $depth);
                }
            }
        }

        return $arrayKeys;
    }
?>

EXAMPLE:
input:
array(
    'Player' => array(
        'id' => '4',
        'state' => 'active',
    ),
    'LevelSimulation' => array(
        'id' => '1',
        'simulation_id' => '1',
        'level_id' => '1',
        'Level' => array(
            'id' => '1',
            'city_id' => '8',
            'City' => array(
                'id' => '8',
                'class' => 'home',
            )
        )
    ),
    'User' => array(
        'id' => '48',
        'gender' => 'M',
        'group' => 'user',
        'username' => 'Hello'
    )
)

output:
array(
    'Player' => array(),
    'LevelSimulation' => array(
        'Level' => array(
            'City' => array()
        )
    ),
    'User' => array()
)

down

Paul Hirsch ¶

9 years ago

It is worth noting that array_keys does not maintain the data-type of the keys when mapping them to a new array.  This created an issue with in_array and doing  a lookup on characters from a string.  NOTE:  my lookup $array has a full map of numbers and characters - upper and lower - to do an simple faux encryption with.

<?php
$array = array(
     'e' => 'ieio'
    ,'1' => 'one'
    ,'2' => 'two'
    ,'0' => 'zero'
);
var_dump($array);
$keys = array_keys($array);
var_dump($keys);

$string = '1e0';
for ($i = 0; $i < strlen($string); $i++) {
    if (in_array($string[$i],$keys,'strict')) echo 'dude ';
    else echo 'sweet ';
}
?>

Outputs:
array (size=4)
  'e' => string 'ieio' (length=4)
  1 => string 'one' (length=3)
  2 => string 'two' (length=3)
  0 => string 'zero' (length=4)

array (size=4)
  0 => string 'e' (length=1)
  1 => int 1
  2 => int 2
  3 => int 0

sweet dude sweet 

----  
expected to see:
dude dude dude

down

zammit dot andrew at gmail dot com ¶

11 years ago

If an array is empty (but defined), or the $search_value is not found in the array, an empty array is returned (not false, null, or -1). This may seem intuitive, especially given the documentation says an array is returned, but I needed to sanity test to be sure:

<?php

$emptyArray = array();
var_dump(array_keys($emptyArray,99)); // array (size=0) \ empty

$filledArray = array(11,22,33,42);
var_dump(array_keys($filledArray,99)); // array (size=0) \ empty

?>

down

Robert C. ¶

8 years ago

Keys from multi dimensional array to simple array

Want to traverse an multi dimensional array and get the keys back in a single dimensional array? This will do the trick:

<?php

    public function array_walk_keys($array, $parentKey = null, &$flattened_array = null)
    {
        if(!is_array($array))
            return $array;
        
        foreach( $array as $key => $val ) {
            $flattenedKeysArray[] = $key;
            
            if(is_array($val))
                array_walk_keys($val, $key, $flattenedKeysArray);
        }

        return $flattenedKeysArray;
    }

down

phpnet at holodyn dot com ¶

11 years ago

Since 5.4 STRICT standards dictate that you cannot wrap array_keys in a function like array_shift that attempts to reference the array.  

Invalid:
echo array_shift( array_keys( array('a' => 'apple') ) );

Valid:
$keys = array_keys( array('a' => 'apple') );
echo array_shift( $keys );

But Wait! Since PHP (currently) allows you to break a reference by wrapping a variable in parentheses, you can currently use:

echo array_shift( ( array_keys( array('a' => 'apple') ) ) );

However I would expect in time the PHP team will modify the rules of parentheses.

down

jochem ¶

18 years ago

might be worth noting in the docs that not all associative (string) keys are a like, output of the follow bit of code demonstrates - might be a handy introduction to automatic typecasting in php for some people (and save a few headaches):



<?php

$r = array("0"=>"0","1"=>"1","" =>"2"," "=>"3");

echo 'how php sees this array: array("0"=>"0","1"=>"1","" =>"2"," "=>"3")',"\n-----------\n";

var_dump($r); print_r($r); var_export($r);

echo "\n-----------\n",'var_dump("0","1",""," ") = ',"\n-----------\n";

var_dump("0","1",""," ");

?>



OUTPUTS:



how php sees this array: array("0"=>"0","1"=>"1","" =>"2"," "=>"3")

-----------

array(4) {

  [0]=>

  string(1) "0"

  [1]=>

  string(1) "1"

  [""]=>

  string(1) "2"

  [" "]=>

  string(1) "3"

}

Array

(

    [0] => 0

    [1] => 1

    [] => 2

    [ ] => 3

)

array (

  0 => '0',

  1 => '1',

  '' => '2',

  ' ' => '3',

)

-----------

var_dump("0","1",""," ") =

-----------

string(1) "0"

string(1) "1"

string(0) ""

string(1) " "

down

vittorh at outlook dot com ¶

1 year ago

Hello friends

Guys in the array_keys function manual - https://www.php.net/manual/pt_BR/function.array-keys.php

it is described that in the second search parameter " $filter_value " is defined in mixed type, however in php 7.4 to version 8.1 when passing a $filter_value of type array, the function without returning false ( empty array )

Exemple : 

$array = ['a' => 1, 'b' => 2];

var_dump(array_keys($array,array('a','b'))); // return array(0) { }
var_dump(array_keys($array,array('a','b'))); // return array(0) { }
var_dump(array_keys($array,'a')); // return array(0) { }

Debugging the code, I believe that the description of the $filter_value parameter is confusing, as it is understood that the value of the key in the array has to be the specified value and not the content of the key

down

el dot quick at gmail dot com ¶

13 years ago

Sorry for my english...



I wrote a function to get keys of arrays recursivelly...



<?php

    function recursive_keys($input, $search_value = null){



        $output = ($search_value !== null ? array_keys($input, $search_value) : array_keys($input)) ;

        foreach($input as $sub){

            if(is_array($sub)){

                $output = ($search_value !== null ? array_merge($output, recursive_keys($sub, $search_value)) : array_merge($output, recursive_keys($sub))) ;

            }

        }

        return $output ;

    }

?>



I hope it will be usefull



Regards

down

enrique dot balleste at gmail dot com ¶

7 years ago

A nice little trick to get all of the keys who have some type of value:

$keys = array_keys($array, !null);

down

ayyappan dot ashok at gmail dot com ¶

8 years ago

Post By  Sven (59892) has to be changed

$array = array('first'=>'111', 'second'=>'222', 'third'=>'333');

$rarray = array_keys($array);

print array_shift($rarray); // first

print array_pop($rarray); //thrid

print array_shift($rarray); //second

print array_pop($rarray); // no result

Code below is not valid from 5.4.0 

print array_shift(array_keys($array)); Throws Strict Standards Error

Sven code works fine till 5.3.29. From 5.4.0  the standards have been changed and results too differ.

Note :
Strict Standards :  Only variables should be passed by reference

Please have look of the code in different versions

http://sandbox.onlinephpfunctions.com/code/24b5fddf14b635f1e37db69a7edffc2cbbed55e1

http://sandbox.onlinephpfunctions.com/code/f695e8f81e906b4f062b66cf9b3b83b6b620464c

down

ferrerna at gmail dot com ¶

15 years ago

Here's a function I needed to collapse an array, in my case from a database query.  It takes an array that contains key-value pairs and returns an array where they are actually the key and value.

<?php

function array_collapse($arr, $x, $y) {
    $carr = array();
    while ($el = current($arr)) {
        $carr[ $el[$x] ] = $el[$y];
        next($arr);
    }
    return $carr;
}

?>

Example usage (pseudo-database code):

<?php

$query = db_query('SELECT name, value FROM properties');

$result = db_returnAll($query);

/* This will return an array like so:

[
   ['name' -> 'color', 'value' -> 'blue'],
   ['name' -> 'style', 'value' -> 'wide-format'],
   ['name' -> 'weight', 'value' -> 3.6],
   ['name' -> 'name', 'value' -> 'Waerdthing']
]

*/

$propArr = array_collapse($result, 'name', 'value');

/* Now this array looks like:

[
   ['color' -> 'blue'],
   ['style' -> 'wide-format'],
   ['weight' -> 3.6],
   ['name' -> 'Waerdthing'],

*/

?>

I found this handy for using with json_encode and am using it for my project http://squidby.com

down

rodrigo at NOSPAM dot dhweb dot com dot br ¶

21 years ago

[Editor's note: For a complete solution to the printing of complex structures or hashes, see the PEAR::Var_Dump package: http://pear.php.net/package-info.php?pacid=103 , use "pear install Var_Dump" to get it]



This function will print all the keys of a multidimensional array in html tables.

It will help to debug when you don?t have control of depths.



<?php

function show_keys($ar){



   echo "<table width='100%' border='1' bordercolor='#6699CC' cellspacing='0' cellpadding='5'><tr valign='top'>";



      foreach ($ar as $k => $v ) {



         echo "<td align='center' bgcolor='#EEEEEE'>

           <table border='2' cellpadding='3'><tr><td bgcolor='#FFFFFF'><font face='verdana' size='1'>

              " . $k . "

           </font></td></tr></table>";



           if (is_array($ar[$k])) {

              show_keys ($ar[$k]);

         }



         echo "</td>";



      }



   echo "</tr></table>";



}



// Multidimensional array ->

$arvore = array();

$arvore['1'] = array();

$arvore['1']['1.1'] = array('1.1.1', '1.1.2', '1.1.3');

$arvore['1']['1.2'] = array('1.2.1', '1.2.2', '1.2.3');

$arvore['1']['1.3'] = array('1.3.1', '1.3.2', '1.3.3');

$arvore['2'] = array();

$arvore['2']['2.1'] = array('2.1.1', '2.1.2', '2.1.3');

$arvore['2']['2.2'] = array('2.2.1', '2.2.2', '2.2.3');

$arvore['2']['2.3'] = array('2.3.1', '2.3.2', '2.3.3');

$arvore['3'] = array();

$arvore['3']['3.1'] = array('3.1.1', '3.1.2', '3.1.3');

$arvore['3']['3.2'] = array('3.2.1', '3.2.2', '3.2.3');

$arvore['3']['3.3'] = array('3.3.1', '3.3.2'=>array('3.3.2.1', '3.3.2.2'), '3.3.3');

// <-



show_keys($arvore);

?>

down

sip at email dot ee ¶

21 years ago

Note, that using array_key_exists() is rather inefficient. The overhead associated with calling a function makes it slower, than using isset($array[$key]), instead of array_key_exists($key, $array)
using isset() is usually about 1.3 times faster, according to my tests.

down

ru dot dy at gmx dot net ¶

19 years ago

I was looking for a function that simply unset a variable amout of values from a one-dimensional array by key. I ended up with this (returns the array itself if no further parameter than the array is given, false with no params - does not change the source array) 

usage: array_remove(array $input [, mixed key ...])

<?php

  function array_remove() {
    if ($stack = func_get_args()) {
      $input = array_shift($stack);
      foreach ($stack as $key) {
        unset($input[$key]);
      }
      return $input;
    }
    return false;
  }

?>

Test:

<?php
  $a = array('a'=>'fun', 'b'=>3.14, 'sub'=> array('1', '2', '3'), 'd'=>'what', 'e' => 'xample', 5 => 'x');
  print_r($a);
  print_r(array_remove($a, 'd', 'b', 5, 'sub'));
?>

Output: 

Array
(
    [a] => fun
    [b] => 3.14
    [sub] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [d] => what
    [e] => xample
    [5] => x
)
Array
(
    [a] => fun
    [e] => xample
)

Hope this helps someone.

down

Md. Abutaleb ¶

4 years ago

<?php  
# array_keys() also return the key if it's boolean but the boolean will return as 1 or 0. It will return empty if get NULL value as key. Consider the following array: 

$a = array(
      "first_index" => "This is the first element", 
      true => 3, 
      false => 2, 
      4.5 => 'Something', 
      "08" => 5, 
      "8" => 6, 
      NULL => 'Null key'
    );

print_r(array_keys($a));

Array
(
    [0] => first_index
    [1] => 1
    [2] => 0
    [3] => 4
    [4] => 08
    [5] => 8
    [6] => 
)

?>

down

izzecold at live dot de ¶

12 years ago

<?php

/* A Function created by myself for checking multiple array keys
For Example u got an Array like $_SESSION and u wanna know if the keys 'user','pass','email' and 'type' exists.

*/

function mKeyChecker($arr,$keys=array()) {
    if(count($keys) > 1) {
        $valid_keys = 0;
        foreach($keys as $key) {
            if(array_key_exists($key,$arr)) $valid_keys++;
        }
        if($valid_keys == count($keys)) {
            return true;
        } else {
            return false;
        }
    } else if(count($keys) == 1) {
        if(array_key_exists($key[0],$arr)) {
            return true;
        } else {
            return false;
        }
    } else {
        return false;
    }
}

// Execution Example

if(mKeyChecker($_SESSION,array('id','user','email','type'))) {
    echo "is!";
} else {
    echo "not!";    
}

?>

down

Hayley Watson ¶

17 years ago

An alternative to RQuadling at GMail dot com's array_remove() function:

<?php
function array_remove(array $array, $value, $strict=false)
{
    return array_diff_key($array, array_flip(array_keys($array, $value, $strict)));
}
?>

down

vesely at tana dot it ¶

19 years ago

The position of an element.

One can apply array_keys twice to get the position of an element from its key. (This is the reverse of the function by cristianDOTzuddas.) E.g., the following may output "yes, we have bananas at position 0".

<?php
$a = array("banana" => "yellow", "apple" = "red");
$k = get_some_fruit();
if (isset($a[$k]))
{
   list($pos) = array_keys(array_keys($a), $k);
   print "yes, we have {$k}s at position $pos\n";
}
?>

Not amazingly efficient, but I see no better alternative.

down

creator at mindcreations dot com ¶

17 years ago

This function will extract keys from a multidimensional array



<?php

function multiarray_keys($ar) {

            

    foreach($ar as $k => $v) {

        $keys[] = $k;

        if (is_array($ar[$k]))

            $keys = array_merge($keys, multiarray_keys($ar[$k]));

    }

    return $keys;

}

?>



Example code:



<?php

$array = array("color" => array("1stcolor" => "blue", "2ndcolor" => "red", "3rdcolor" => "green"),

               "size"  => array("small", "medium", "large"));



echo "<pre>";

print_r($array);

echo "</pre>";



echo "<pre>";

print_r(multiarray_keys($array));

echo "</pre>";

?>



Example output:



Array

(

    [color] => Array

        (

            [1stcolor] => blue

            [2ndcolor] => red

            [3rdcolor] => green

        )



    [size] => Array

        (

            [0] => small

            [1] => medium

            [2] => large

        )



)



Array

(

    [0] => color

    [1] => 1stcolor

    [2] => 2ndcolor

    [3] => 3rdcolor

    [4] => size

    [5] => 0

    [6] => 1

    [7] => 2

)

down

craig1231 at hotmail dot com ¶

12 years ago

A needed a function to find the keys which contain part of a string, not equalling a string...

<?php
function array_keys_contain($input, $search_value, $strict = false)
    {
        $tmpkeys = array();

        $keys = array_keys($input);

        foreach ($keys as $k)
        {
            if ($strict && strpos($k, $search_value) !== FALSE)
                $tmpkeys[] = $k;
            elseif (!$strict && stripos($k, $search_value) !== FALSE)
                $tmpkeys[] = $k;
        }

        return $tmpkeys;
    }
?>

down

webmaster [at] baz-x [dot] at ¶

19 years ago

I was looking for a function that deletes either integer keys or string keys (needed for my caching).
As I didn't find a function I came up with my own solution.
I didn't find the propiest function to post to so I will post it here, hope you find it useful.

<?php

function array_extract($array, $extract_type = 1)
{
    foreach ( $array as $key => $value )
    {
        if ( $extract_type == 1 && is_string($key) )
        {
            // delete string keys
            unset($array[$key]);
        }
        elseif ( $extract_type == 2 && is_int($key) )
        {
            // delete integer keys
            unset($array[$key]);
        }
    }

    return $array;
}

?>

You can of course define constants to have a nicer look, I have chosen these: EXTR_INT = 1; EXTR_STRING = 2
EXTR_INT will return an array where keys are only integer while
EXTR_STRING will return an array where keys are only string

Have fun with it.

down

theking2 at king dot ma ¶

1 month ago

PHP arrays are weird. So the result of

<?php
$array = [
    'element0',
    'element1' => 'Hello World',
    'element2' => 42,
    'element3' => [ 'aaa', 'bbb' ],
];
print_r( array_keys($array) );
?>
is a surprising though correct
Array
(
    [0] => 0
    [1] => element1
    [2] => element2
    [3] => element3
)

Arrays in php are really ordered key value pairs where if a value is missing it will become a value with a number as key. To get the intuitive first dimension of an array use a map with function to determine if a key is in fact an index.
This

<?php
$array_first = array_map(
    fn ($key)=>
        is_int($key)?$array[$key]:$key
    ,
    array_keys($array)
);
print_r($array_first);
?>

will show

Array
(
    [0] => element0
    [1] => element1
    [2] => element2
    [3] => element3
)

However in a niche case 
<?php
$array = [
    'element0',
    'element1' => 'Hello World',
    'element2' => 42,
    'element3' => [ 'aaa', 'bbb' ],
    12 => 'I\'m a field, not a key'
];
?>
this won't work of course and output
Array
(
    [0] => element0
    [1] => element1
    [2] => element2
    [3] => element3
    [4] => I'm a field, not a key
)

down

qeremy [atta] gmail [dotta] com ¶

12 years ago

Simple ways to prefixing arrays;

<?php
function array_keys_prefix($arr, $pref = "") {
    $rarr = array();
    foreach ($arr as $key => $val) {
        $rarr[$pref.$key] = $val;
    }
    return $rarr;
}

function array_keys_prefix_multi($arr, $pref = "") {
    $rarr = array();
    foreach ($arr as $key => $val) {
        $rarr[] = array_keys_prefix($val, $pref);
    }
    return $rarr;
}

// $a = array("foo" => "FOO", "bar" => "BAR", "baz" => "BAZ"); // or
$a = array("foo" => "FOO", "bar" => "BAR", "baz" => array(1,2,3));
print_r(array_keys_prefix($a, "my_"));

// db fetch...
$products = array(
    array("id" => 1, "name" => "Foo"),
    array("id" => 2, "name" => "Bar")
);
print_r(array_keys_prefix_multi($products, "product_"));
?>

Array
(
    [my_foo] => FOO
    [my_bar] => BAR
    [my_baz] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

)
Array
(
    [0] => Array
        (
            [product_id] => 1
            [product_name] => Foo
        )

    [1] => Array
        (
            [product_id] => 2
            [product_name] => Bar
        )

)

down

glennh at webadept dot net ¶

22 years ago

All the cool notes are gone from the site. 



Here's an example of how to get all the variables passed to your program using the method on this page. This prints them out so you can see what you are doing. 



<?php

while (list($key, $value) = each

(${"HTTP_".$REQUEST_METHOD."_VARS"}))

{

        echo $key." = ".$value." ";

}

?>

down

-2

neil at 11 out of 10 ¶

16 years ago

<?php

/*
 * This function will return the keys of elements in the 
 * haystack where the value is found in array needle
 */

function array_value_intersect_keys( $array_haystack, $array_needle ){
    $intersected = array_intersect( $array_haystack, $array_needle );
    return array_keys( $intersected );
}

// usage 

$array_haystack = array( 1 => 2, 2 => 5, 'red' => 8, 9 => 14 );

$array_needle = array( 2, 8 );

$array_keys_of_intersecting_values = array_value_intersect_keys( $array_haystack, $array_needle );

print_r( $array_keys_of_intersecting_values );
?>

returns
Array
(
    [0] => 1
    [1] => red
)

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array_keys

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