mb_substr

(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)

mb_substr — 获取部分字符串

说明

mb_substr(
    string $string,
    int $start,
    ?int $length = null,
    ?string $encoding = null
): string

根据字符数执行一个多字节安全的 substr() 操作。位置是从 string 的开始位置进行计数。第一个字符的位置是 0。第二个字符的位置是 1，以此类推。

参数

string

从该 string 中提取子字符串。

start

如果 start 不是负数，返回的字符串会从 string 第 start 的位置开始，从 0 开始计数。举个例子，字符串 'abcdef'，位置 0 的字符是 'a'，位置 2 的字符是 'c'，以此类推。

如果 start 是负数，返回的字符串是从 string 末尾处第 start 个字符开始的。

length

string 中要使用的最大字符数。如果省略了此参数或者传入了 NULL，则会提取到字符串的尾部。

encoding

encoding 参数为字符编码。如果省略或是 null，则使用内部字符编码。

返回值

mb_substr() 函数根据 start 和 length 参数返回 string 中指定的部分。

更新日志

版本	说明
8.0.0	现在 `encoding` 可以为 null。

参见

mb_strcut() - 获取字符的一部分
mb_internal_encoding() - 设置/获取内部字符编码

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用户贡献的备注 9 notes

down

qbolec at gmail dot com ¶

9 years ago

As you often need to iterate over UTF-8 characters inside a string, you might be tempted to use mb_substr($text,$i,1).
The problem with this is that there is no "magic" way to find $i-th character inside UTF-8 string, other than reading it byte by byte from the begining. Thus a loop which calls mb_substr($text,$i,1) N times for all possible N values of $i, will take much longer than expected. The larger the $i gets, the longer is the search for $i-th letter. As characters are between 1 to 6 bytes long, one can convince oneself, that the execution time of such loop is actually Theta(N^2), which can be really slow even for moderately long texts.
One way to work around it is to first split your text into an array of letters using some smart preprocessing, and only then iterate over the array.
Here is the idea:
<?php
class Strings
{
  public static function len($a){
    return mb_strlen($a,'UTF-8');
  }
  public static function charAt($a,$i){
    return self::substr($a,$i,1);
  }
  public static function substr($a,$x,$y=null){
    if($y===NULL){
      $y=self::len($a);
    }
    return mb_substr($a,$x,$y,'UTF-8');
  }
  public static function letters($a){
    $len = self::len($a);
    if($len==0){
      return array();
    }else if($len == 1){
      return array($a);
    }else{
      return Arrays::concat(
        self::letters(self::substr($a,0,$len>>1)),
        self::letters(self::substr($a,$len>>1))
      );
    }
  }
?>
As you can see, the Strings::letters($text) split the text recursively into two parts. Each level of the recursion requires time linear in the length of the string, and there is logarithmic number of levels, so the total runtime is O(N log N), which is still more than theoretically optimal O(N), but sadly this is the best idea I've got.

down

drraf at tlen dot pl ¶

19 years ago

Note: If borders are out of string - mb_string() returns empty _string_, when function substr() returns _boolean_ false in this case.
Keep this in mind when using "===" comparisions.

Example code:
<?php

var_dump( substr( 'abc', 5, 2 ) ); // returns "false"
var_dump( mb_substr( 'abc', 5, 2 ) ); // returns ""

?>

It's especially confusing when using mbstring with function overloading turned on.

down

p dot assenov at aip-solutions dot com ¶

12 years ago

I'm trying to capitalize only the first character of the string and tried some of the examples above but they didn't work. It seems mb_substr() cannot calculate the length of the string in multi-byte encoding (UTF-8) and it should be set explicitly. Here is the corrected version:



<?php

function mb_ucfirst($str, $enc = 'utf-8') {

    return mb_strtoupper(mb_substr($str, 0, 1, $enc), $enc).mb_substr($str, 1, mb_strlen($str, $enc), $enc);

}

?>



cheers!

down

xiaogil at yahoo dot fr ¶

19 years ago

Thanks Darien from /freenode #php for the following example (a little bit changed).

It just prints the 6th character of $string.
You can replace the digits by the same in japanese, chinese or whatever language to make a test, it works perfect.

<?php
mb_internal_encoding("UTF-8");
$string = "0123456789";
$mystring = mb_substr($string,5,1);
echo $mystring;
?>

(I couldn't replace 0123456789 by chinese numbers for example here, because it's automatically converted into latin digits on this website, look : 
&#38646;&#19968;&#20108;&#19977;&#22235;
&#20116;&#20845;&#19971;&#20843;&#20061;)

gilv

down

public at luedi dot jp ¶

7 months ago

Just wanted to add that not only the `start` can be a negative number, the `length` can be negative, too. And it works as expected.

mb_substr( "1234567890", 3, -4, "UTF-8" ) => "456".

So it cuts off the last 4 characters.

down

boulahdidraid18 at gmail dot com ¶

10 months ago

Here is an example demonstrating the difference between `substr` and `mb_substr` functions:

1- When working with non UTF-8 characters, both functions behave the same and give the same output:

$str = 'abcdef';
echo substr($s, 0, 3); // abc
echo mb_substr($s, 0, 3); // abc

2- When working with UTF-8 characters, each function will behave differently and give a different result:

2.A- The 'substr' function works on the byte-level and with single-byte encoded characters only (doesn't support multibyte encoding). 

For example:

$str_utf8 = utf8_encode("déjà_vu");

If we do this:

echo substr($str_utf8, 0, 3); // dé
echo substr($str_utf8, 0, 2); // d�

=> That's because the special character "é" (and "à") is internally coded with two bytes:

PHP will start reading the first byte at index 0, which represents `d`, then move to the second byte, which is a part of the two-byte encoding of the character ` é `,  and since the length is set to 2, PHP will stop here and doesn't continue reading the third byte, so it doesn't recognize the character ` é ` and prints � instead of é.

2.B- The 'mb_substr' function works on the character-level and supports multibyte encoded characters. This means, PHP counts the number of characters only and doesn't take into consideration the number of bytes of their encoding, for example:

$str_utf8 = utf8_encode("déjà_vu");

echo mb_substr($str_utf8, 0, 4, "UTF-8"); // Déjà
echo mb_substr($str_utf8, 1, 4, "UTF-8"); // éjà_
echo mb_substr($str_utf8, 6, 4, "UTF-8"); // u
echo mb_substr($str_utf8, 7, 4, "UTF-8"); // ''
echo mb_substr($str_utf8, -2, "UTF-8"); // vu
echo mb_substr($str_utf8, -2, 1, "UTF-8"); // v
echo mb_substr($str_utf8, -2, 3, "UTF-8"); // vu

down

desmatic at gmail dot com ¶

11 years ago

quick and dirty loop through multibyte string
<?php
function get_character_classes($string, $encoding = "UTF-8") {
    $current_encoding = mb_internal_encoding();
    mb_internal_encoding($encoding);
    $has = array();
    $stringlength = mb_strlen($string, $encoding);
    for ($i=0; $i < $stringlength; $i++) {
        $c = mb_substr($string, $i, 1);
        if (($c >= "0") && ($c <= "9")) {
            $has['numeric'] = "numeric";
        } else if (($c >= "a") && ($c <= "z")) {
            $has['alpha'] = "alpha";
            $has['alphalower'] = 'alphalower';
        } else if (($c >= "A") && ($c <= "Z")) {
            $has['alpha'] = "alpha";
            $has['alphaupper'] = "alphaupper";
        } else if (($c == "$") || ($c == "£")) {
            $has['currency'] = "currency";
        } else if (($c == ".") && ($has['decimal'])) {
            $has['decimals'] = "decimals";
        } else if ($c == ".") {
            $has['decimal'] = "decimal";
        } else if ($c == ",") {
            $has['comma'] = "comma";
        } else if ($c == "-") {
            $has['dash'] = "dash";
        } else if ($c == " ") {
            $has['space'] = "space";
        } else if ($c == "/") {
            $has['slash'] = "slash";
        } else if ($c == ":") {
            $has['colon'] = "colon";
        } else if (($c >= " ") && ($c <= "~")) {
            $has['ascii'] = "ascii";
        } else {
            $has['binary'] = "binary";
        }
    }
    mb_internal_encoding($current_encoding);
    
    return $has;
}

$string = "1234asdfA£^_{}|}~žščř";
echo print_r(get_character_classes($string), true);
?>

Array
(
    [numeric] => numeric
    [alpha] => alpha
    [alphalower] => alphalower
    [alphaupper] => alphaupper
    [currency] => currency
    [ascii] => ascii
    [binary] => binary
)

down

-3

sanjuro at 1up-games dot com ¶

11 years ago

A serious pitfall when using mb_substr() set to HTML-ENTITIES encoding is that the function performs a number of conversions before returning the value, the worst one being that html special characters are not just counted but decoded.

<?php

mb_internal_encoding("ISO-8859-1"); echo mb_internal_encoding(),"\n<br><br>\n";

$a='j&uuml;st &#228; &quot; simple &quot; &#26085;&#26412; &lt;b&gt;test&lt;/b&gt;';

echo mb_substr($a,0),"\n<br><br>\n";
// page source: j&uuml;st &#228; &quot; simple &quot; &#26085;&#26412; &lt;b&gt;test&lt;/b&gt;

echo mb_substr($a,0,strlen($a),'HTML-ENTITIES');
// page source: j&uuml;st &auml; " simple " &#26085;&#26412; <b>test</b>

?>

down

-5

qdinar at gmail dot com ¶

8 years ago

you can make mb_substr working faster with long strings with usage of ucs-2 encoding.

<?php

header('Content-Type: text/html; charset=utf-8');
echo '<meta http-equiv="Content-Type" content="text/html; charset=utf-8" >';

function test($string, $encoding='utf8'){
    $t1=microtime(true);
    $textlen=mb_strlen($string);
    $substr_len=3;
    for($i=0;$i<$textlen-$substr_len+1;$i++){
        $substr=mb_substr($string,$i,$substr_len);
    }
    echo 'mb_substr, '.$encoding.': '.(microtime(true)-$t1);
    echo ' . check: ';
    if($encoding=='ucs2'){
        $substr=mb_convert_encoding($substr,'utf-8','ucs2');
    }
    var_dump( $substr );
    echo ' . <br>';
    echo '<br>';
}

$corpus_short=str_repeat('тест Тест ',1000);
// it works likewise slowly with "test Test" with utf8
mb_internal_encoding('utf-8');
test($corpus_short);

$corpus_short_ucs2=mb_convert_encoding($corpus_short,'ucs2','utf-8');
mb_internal_encoding('ucs2');
test($corpus_short_ucs2,'ucs2');

?>

output:

mb_substr, utf8: 0.26480984687805 . check: string(5) "ст " .

mb_substr, ucs2: 0.0048871040344238 . check: string(5) "ст " .