lottery.php

<?php

$n = 50;
$k = 7;

function boom($var) {
	$return = 1;
	
	while ($var > 0) {
		$return = ($return * $var);
		--$var;
	}

	return (double)$return;
}

$_k = boom($k);
$_n = boom($n);
$_p = boom($n - $k);

// n! / (k! * (n - k)!)
$prob = ($_n / ($_k * $_p));

echo $prob;