A little problem with array checksums using PHP

I was recently working at Array Checksums problem with PHP and I'm really desperated to do this.

I've checked topic about checksums in codeabbey wiki still nothing.

Here's the code:

<?php
$str = "1683 744645104 4323217 5987 15058 4854656 2 518452182 494 37869284 145 8166 75204 817 931 683178918 9 8620 771 81279 227701598";
$a = explode(" ", $str);
$limit = 10000007;
$r = 0;
for ($i = 0; $i < count($a); $i++) {
$r = (($r + $a[$i]) * 113); 
if ($r > $limit) {
    $r = $r % $limit;
}
}
echo $r;
?>

This script returns value of "-3.1509750703419E+33".

Example (3 1 4 1 5 9) runs fine and returns correct value.

Any ideas, please?

Thanks Eugene, finally it works. I'm confused now, because I've tried it before posting and it didn't work.

Maybe I was just too tired and made some stupid mistakes.