Inner union of strings by the largest common part

function intersect_str($left, $right)
{
    $str1 = str_split($left);
    $str2 = str_split($right);

    $number_of_common_characters = 0;

    $len1 = count($str1);
    $len2 = count($str2);

    for ($i = 0; $i < ($len1 > $len2 ? $len2 : $len1); $i++) {
        if (substr($left, $len1 - 1 - $i, $i + 1) == substr($right, 0, $i + 1)) {
            $number_of_common_characters = $i + 1;
        }
    }
    return $left . substr($right, $number_of_common_characters, $len2 - $number_of_common_characters);
}