(PHP 3 >= 3.0.17, PHP 4 >= 4.0.1, PHP 5)
levenshtein 
Calculate Levenshtein distance between two strings
Description
int
levenshtein ( string str1, string str2 [, int cost_ins, int cost_rep, int cost_del] )
This function returns the LevenshteinDistance between the
two argument strings or 1, if one of the argument strings
is longer than the limit of 255 characters.
The Levenshtein distance is defined as the minimal number of
characters you have to replace, insert or delete to transform
str1 into str2.
The complexity of the algorithm is O(m*n),
where n and m are the
length of str1 and
str2 (rather good when compared to
similar_text(), which is O(max(n,m)**3),
but still expensive).
In its simplest form the function will take only the two
strings as parameter and will calculate just the number of
insert, replace and delete operations needed to transform
str1 into str2.
A second variant will take three additional parameters that
define the cost of insert, replace and delete operations. This
is more general and adaptive than variant one, but not as
efficient.
Example 1. levenshtein() example
<?php // input misspelled word $input = 'carrrot';
// array of words to check against $words = array('apple','pineapple','banana','orange', 'radish','carrot','pea','bean','potato');
// no shortest distance found, yet $shortest = 1;
// loop through words to find the closest foreach ($words as $word) {
// calculate the distance between the input word, // and the current word $lev = levenshtein($input, $word);
// check for an exact match if ($lev == 0) {
// closest word is this one (exact match) $closest = $word; $shortest = 0;
// break out of the loop; we've found an exact match break; }
// if this distance is less than the next found shortest // distance, OR if a next shortest word has not yet been found if ($lev <= $shortest  $shortest < 0) { // set the closest match, and shortest distance $closest = $word; $shortest = $lev; } }
echo "Input word: $input\n"; if ($shortest == 0) { echo "Exact match found: $closest\n"; } else { echo "Did you mean: $closest?\n"; }
?>

The above example will output: Input word: carrrot
Did you mean: carrot? 

See also soundex(),
similar_text(), and
metaphone().