# similar_text

(PHP 4, PHP 5, PHP 7)

similar_textCalculate the similarity between two strings

### Description

similar_text ( string \$first , string \$second [, float &\$percent ] ) : int

This calculates the similarity between two strings as described in Programming Classics: Implementing the World's Best Algorithms by Oliver (ISBN 0-131-00413-1). Note that this implementation does not use a stack as in Oliver's pseudo code, but recursive calls which may or may not speed up the whole process. Note also that the complexity of this algorithm is O(N**3) where N is the length of the longest string.

### Parameters

first

The first string.

second

The second string.

Note:

Swapping the first and second may yield a different result; see the example below.

percent

By passing a reference as third argument, similar_text() will calculate the similarity in percent, by dividing the result of similar_text() by the average of the lengths of the given strings times 100.

### Return Values

Returns the number of matching chars in both strings.

The number of matching characters is calculated by finding the longest first common substring, and then doing this for the prefixes and the suffixes, recursively. The lengths of all found common substrings are added.

### Examples

Example #1 similar_text() argument swapping example

This example shows that swapping the first and second argument may yield different results.

<?php
\$sim
similar_text('bafoobar''barfoo'\$perc);
echo
"similarity: \$sim (\$perc %)\n";
\$sim similar_text('barfoo''bafoobar'\$perc);
echo
"similarity: \$sim (\$perc %)\n";

The above example will output something similar to:

similarity: 5 (71.428571428571 %)
similarity: 3 (42.857142857143 %)

• levenshtein() - Calculate Levenshtein distance between two strings
• soundex() - Calculate the soundex key of a string

### User Contributed Notes 11 notes

80
SPAM HATER
6 years ago
Hey there,

Be aware when using this function, that the order of passing the strings is very important if you want to calculate the percentage of similarity, in fact, altering the variables will give a very different result, example :

<?php
\$var_1
= 'PHP IS GREAT';
\$var_2 = 'WITH MYSQL';

similar_text(\$var_1, \$var_2, \$percent);

echo
\$percent;
// 27.272727272727

similar_text(\$var_2, \$var_1, \$percent);

echo
\$percent;
// 18.181818181818
?>
68
daniel dot karbach at localhorst dot tv
8 years ago
Please note that this function calculates a similarity of 0 (zero) for two empty strings.

<?php
similar_text
("", "", \$sim);
echo
\$sim; // "0"
?>
17
vasyl at vasyltech dot com
3 years ago
Recursive algorithm usually is very elegant one. I found a way to get better precision without the recursion. Imagine two different (or same) length ribbons with letters on each. You simply shifting one ribbon to left till it matches the letter the first.

<?php

function similarity(\$str1, \$str2) {

\$len1 = strlen(\$str1);

\$len2 = strlen(\$str2);

\$max = max(\$len1, \$len2);

\$similarity = \$i = \$j = 0;

while ((
\$i < \$len1) && isset(\$str2[\$j])) {
if (
\$str1[\$i] == \$str2[\$j]) {

\$similarity++;

\$i++;

\$j++;
} elseif (
\$len1 < \$len2) {

\$len1++;

\$j++;
} elseif (
\$len1 > \$len2) {

\$i++;

\$len1--;
} else {

\$i++;

\$j++;
}
}

return
round(\$similarity / \$max, 2);
}

\$str1 = '12345678901234567890';
\$str2 = '12345678991234567890';

echo
'Similarity: ' . (similarity(\$str1, \$str2) * 100) . '%';
?>
13
ryan at derokorian dot com
5 years ago
Note that this function is case sensitive:

<?php

\$var1
= 'Hello';
\$var2 = 'Hello';
\$var3 = 'hello';

echo
similar_text(\$var1, \$var2);  // 5
echo similar_text(\$var1, \$var3);  // 4
Paul
12 years ago
The speed issues for similar_text seem to be only an issue for long sections of text (>20000 chars).

I found a huge performance improvement in my application by just testing if the string to be tested was less than 20000 chars before calling similar_text.

20000+ took 3-5 secs to process, anything else (10000 and below) took a fraction of a second.
Fortunately for me, there was only a handful of instances with >20000 chars which I couldn't get a comparison % for.
daniel at reflexionsdesign dot com
17 years ago
If performance is an issue, you may wish to use the levenshtein() function instead, which has a considerably better complexity of O(str1 * str2).
julius at infoguiden dot no
16 years ago
If you have reserved names in a database that you don't want others to use, i find this to work pretty good.
I added strtoupper to the variables to validate typing only. Taking case into consideration will decrease similarity.

<?php
\$query
= mysql_query("select * from \$table") or die("Query failed");

while (
\$row = mysql_fetch_array(\$query)) {

similar_text(strtoupper(\$_POST['name']), strtoupper(\$row['reserved']), \$similarity_pst);
if (
number_format(\$similarity_pst, 0) > 90){

\$too_similar = \$row['reserved'];
print
"The name you entered is too similar the reserved name &quot;".\$row['reserved']."&quot;";
break;
}
}
?>
I_HATE_SPAMMER- PAZ!
4 years ago
it works good. But before processing i think is a good way to make a little mod like this

\$var_1 = strtoupper("doggy");
\$var_2 = strtoupper("Dog");

similar_text(\$var_1, \$var_2, \$percent);

echo \$percent; // output is 75 but without strtoupper output is 50
georgesk at hotmail dot com
17 years ago
Well, as mentioned above the speed is O(N^3), i've done a longest common subsequence way that is O(m.n) where m and n are the length of str1 and str2, the result is a percentage and it seems to be exactly the same as similar_text percentage but with better performance... here's the 3 functions i'm using..

<?php
function LCS_Length(\$s1, \$s2)
{

\$m = strlen(\$s1);

\$n = strlen(\$s2);

//this table will be used to compute the LCS-Length, only 128 chars per string are considered

\$LCS_Length_Table = array(array(128),array(128));

//reset the 2 cols in the table

for(\$i=1; \$i < \$m; \$i++) \$LCS_Length_Table[\$i][0]=0;
for(
\$j=0; \$j < \$n; \$j++) \$LCS_Length_Table[0][\$j]=0;

for (
\$i=1; \$i <= \$m; \$i++) {
for (
\$j=1; \$j <= \$n; \$j++) {
if (
\$s1[\$i-1]==\$s2[\$j-1])

\$LCS_Length_Table[\$i][\$j] = \$LCS_Length_Table[\$i-1][\$j-1] + 1;
else if (
\$LCS_Length_Table[\$i-1][\$j] >= \$LCS_Length_Table[\$i][\$j-1])

\$LCS_Length_Table[\$i][\$j] = \$LCS_Length_Table[\$i-1][\$j];
else

\$LCS_Length_Table[\$i][\$j] = \$LCS_Length_Table[\$i][\$j-1];
}
}
return
\$LCS_Length_Table[\$m][\$n];
}

function
str_lcsfix(\$s)
{

\$s = str_replace(" ","",\$s);

\$s = ereg_replace("[��������]","e", \$s);

\$s = ereg_replace("[������������]","a", \$s);

\$s = ereg_replace("[��������]","i", \$s);

\$s = ereg_replace("[���������]","o", \$s);

\$s = ereg_replace("[��������]","u", \$s);

\$s = ereg_replace("[�]","c", \$s);
return
\$s;
}

function
get_lcs(\$s1, \$s2)
{

//ok, now replace all spaces with nothing

\$s1 = strtolower(str_lcsfix(\$s1));

\$s2 = strtolower(str_lcsfix(\$s2));

\$lcs = LCS_Length(\$s1,\$s2); //longest common sub sequence

\$ms = (strlen(\$s1) + strlen(\$s2)) / 2;

return ((
\$lcs*100)/\$ms);
}
?>

you can skip calling str_lcsfix if you don't worry about accentuated characters and things like that or you can add up to it or modify it for faster performance, i think ereg is not the fastest way?
hope this helps.
Georges
-1
romain dot boyer at gmail dot com
11 years ago
Like levenchtein(), You can do :

(strlen(\$string2) - similar_text(\$string,\$string2))

to see how much characters have been changed.
-26
donotspamme at mailinator dot com
4 years ago
@I_HATE_SPAMMER- PAZ! (http://php.net/similar_text#115994)
Not only will your code fail the "turkey test" (google it), and for a matter most comments here that use strtoupper()/strtolower(), it also depends very much on the use-case; maybe I wanted the 75% ? I don't see the value of such comments; this is something everybody could've come op with. It's basic programming and turning requirements into algorithms.