max length of border having atleast three non-overlapping occurance in a string

stringBorder.php

<?php

/**
 * For a given string S, a substring b is said to be its border if S starts and end with b.
 * Suppose we need length of longest border b that also occurs in S, even after b is trimmed at ends of S.
 *
 * This function is aimed at that need. ;)
 * 
 */
function maxBorderLen($S){
    $m = 0;
    $l = (int) strlen($S) / 2;
    for ($i = 1; $i < $l; $i++) {
        if ($b = substr($S, 0, $i) and $b == substr($S, 0-$i) 
            and substr_count($S, $b) > 2 // the border has to be atleast three non-overlapping occurance
        ) {
            $m = $i;
        }
    }
    return $m;
}

// Tests:
echo maxBorderLen('abczabczabc'); // 3
echo maxBorderLen('abczbcazabcab'); // 2