Boyer–Moore algorithm implementation in PHP

BoyerMoore.php

<?php

/**
* Returns the index of the first occurrence of the 
* specified substring. If it's not found return -1.
* 
* @param text The string to be scanned
* @param pattern The target string to search
* @return The start index of the substring
*/
function BoyerMoore($text, $pattern) {
    $patlen = strlen($pattern);
    $textlen = strlen($text);
    $table = makeCharTable($pattern);

    for ($i=$patlen-1; $i < $textlen;) { 
        $t = $i;
        for ($j=$patlen-1; $pattern[$j]==$text[$i]; $j--,$i--) { 
            if($j == 0) return $i;
        }
        $i = $t;
        if(array_key_exists($text[$i], $table))
            $i = $i + max($table[$text[$i]], 1);
        else
            $i += $patlen;
    }
    return -1;
}

function makeCharTable($string) {
    $len = strlen($string);
    $table = array();
    for ($i=0; $i < $len; $i++) { 
        $table[$string[$i]] = $len - $i - 1; 
    }
    return $table;
}

excuse me, I have a case study using this method, I need an explanation of each line of code, can you help me?