longest common substring in php, note it's different from longest common subsequence

lcs.php

#!/usr/local/php/bin/php
<?php

function get_longest_common_substring($string_1, $string_2) {
 	$string_1_length = strlen($string_1);
	$string_2_length = strlen($string_2);
	$return          = "";

	if ($string_1_length === 0 || $string_2_length === 0) {
		// No similarities
		return $return;
	}

	$longest_common_subsequence = array();

	// Initialize the CSL array to assume there are no similarities
	for ($i = 0; $i < $string_1_length; $i++) {
		$longest_common_subsequence[$i] = array();
		for ($j = 0; $j < $string_2_length; $j++) {
			$longest_common_subsequence[$i][$j] = 0;
		}
	}

	$largest_size = 0;

	for ($i = 0; $i < $string_1_length; $i++) {
		for ($j = 0; $j < $string_2_length; $j++) {
			// Check every combination of characters
			if ($string_1[$i] === $string_2[$j]) {
				// These are the same in both strings
				if ($i === 0 || $j === 0) {
					// It's the first character, so it's clearly only 1 character long
					$longest_common_subsequence[$i][$j] = 1;
				} else {
					// It's one character longer than the string from the previous character
					$longest_common_subsequence[$i][$j] = $longest_common_subsequence[$i - 1][$j - 1] + 1;
				}

				if ($longest_common_subsequence[$i][$j] > $largest_size) {
					// Remember this as the largest
					$largest_size = $longest_common_subsequence[$i][$j];
					// Wipe any previous results
					$return       = "";
					// And then fall through to remember this new value
				}

				if ($longest_common_subsequence[$i][$j] === $largest_size) {
					// Remember the largest string(s)
					$return = substr($string_1, $i - $largest_size + 1, $largest_size);
				}
			}
			// Else, $CSL should be set to 0, which it was already initialized to
		}
	}

	// Return the list of matches
	return $return;
}