Longest Common Substring in PHP

Longest common substring is a function that can be useful once in a while. Here’s a PHP implementation. Be forewarned, this runs in O(mn) time.

function longest_common_substring($string1, $string2) {
    $L = array();
    $length = 0;
    $pos = 0;
    $array1 =str_split($string1);
    $array2 =str_split($string2);
    foreach ($array1 as $i => $c1) { 
        $L[$i] = array();
        foreach ($array2 as $j => $c2) { 
            $L[$i][$j] = 0;
            if ($c1 == $c2) {
                if ($i == 0 || $j == 0) {
                    // initialize that this character position exists.
                    $L[$i][$j] = 1;
                } else {
                    // increment previous or reset.
                    if (isset($L[$i-1][$j-1])) {
                        $L[$i][$j] = $L[$i-1][$j-1] + 1;
                    } else {
                        $L[$i][$j] = 0;
                    }
                }
                if ($L[$i][$j] > $length) {
                    $length = $L[$i][$j];
                }
                if ((isset($L[$i][$j]))&&($L[$i][$j] == $length)) {
                    $pos = $i;
                }
            }
        }
    }
    if ($length > 0) {
        return substr($string1, $pos - $length + 1, $length);
    } else {
        return '';
    }
}

Usage:

$string1 = 'sadjjasdf this is the string  sdlkjhaskl';
$string2 = 'eriuhysdfnbasi this is the stringbhdjubsdi';

echo longest_common_substring($string1, $string2);