Find longest common parts of two strings

commonParts.php

<?php

    function commonParts($a, $b) {
        $a = trim($a);
        $b = trim($b);

        $str = strlen($a) <= strlen($b) ? strtolower($a) : strtolower($b);
        $pos = 0;
        $max = 0;

        while (strlen($str)) {
            if (!empty(substr($str, $pos, 1)) && strpos($b, substr($str, $pos, 1)) !== FALSE) {
                $part = substr($str, $pos);
                $common = substr($str, $pos, 1);

                while (strlen($part)) {
                    if (strpos($b, $common . substr($part, 0, 1)) !== FALSE && strpos($a, $common . substr($part, 0, 1)) !== FALSE) {
                        $common = $common . substr($part, 0, 1);
                        $pos++;
                    }

                    $part = substr($part, 1);
                }

                $result[] = $common;
                $max = strlen($common) > $max ? strlen($common) : $max;
            }

            $str = substr($str, 1);
        }

        $result = array_filter($result, function($val) use($max) {
            return strlen($val) == $max; 
        });

        // Returns string if found only one common part and array if two or more
        return count($result) == 1 ? array_shift($result) : $result;
    }

    print_r(commonParts('Doors', 'Kangaroo'));