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<?php
//define number
$num = 5;

//initialize variable
$factorial = 1;

//calculate factorial
//by multiplying the number by all the
//numbers between itself and 1
//result: "Factorial of 5 is 120"
for ($x=$num; $x>=1; $x--) {
  $factorial = $factorial * $x;
}
echo "Factorial of $num is $factorial";
?>

A factorial of a number n is the product of all the numbers between n and 1. The easiest way to calculate it is with a for( ) loop which one that starts at n and counts down to 1. Each time the loop runs, the previously calculated product is multiplied by the current value of the loop counter and the result is the factorial of the number n.