Needed to compute some permutations and found the BC extension great but poor on functions, so untill this gets implemented here's the factorial function:

<?php
/* BC FACTORIAL
 * n! = n * (n-1) * (n-2) .. 1 [eg. 5! = 5 * 4 * 3 * 2 * 1 = 120]
 */
function bcfact($n){
    $factorial=$n;
    while (--$n>1) $factorial=bcmul($factorial,$n);
    return $factorial;
}

print bcfact(50); 
//30414093201713378043612608166064768844377641568960512000000000000
?>