a function's meaning

[runeveryday]

i have read over the basic knowledge of php, but still new to it. to improve the skill. i searched some examples to practice it.today,i found one.

http://www.brightyoursite.com/blog/2...gle-page-rank/

but the code for me is hard to understand.

    function StrToNum($Str, $Check, $Magic) {
        	$Int32Unit = 4294967296; 
        
        	$length = strlen($Str);
        	for ($i = 0; $i < $length; $i++) {
        		$Check *= $Magic;
        		
     
        
        		if ($Check >= $Int32Unit) {
        			$Check = ($Check - $Int32Unit * (int) ($Check / $Int32Unit));
        			//if the check less than -2^31
        			$Check = ($Check < -2147483648)? ($Check + $Int32Unit) : $Check;
        		}
        		$Check += ord($Str{$i});
        	}
        	return $Check;
        }

i don't understand it well. expect someon can help me?

1, what's the use and meaning of this line `$Int32Unit = 4294967296;`

2,what's the for loop do? what's the use of this function.

[runeveryday]

anyone helps?