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The factorial of a positive integer is the multiplication of all positive integer less than or equal to that number.

factorial of number n = n! = n(n-1)(n-2)...1

For Example:

5! = 5 × 4 × 3 × 2 × 1 = 120

4! = 4 × 3 × 2 × 1 = 24

Method 1: Using Recursive method

In the below example, a recursive function called factorial() is used to calculate factorial of a number.

<?php
function factorial($x) {
  if ($x == 0 || $x == 1)
    return 1;
  else 
    return $x*factorial($x-1);    
}

echo "10! =  ".factorial(10)."\n";
echo "6! =  ".factorial(6)."\n";
?>

The above code will give the following output:

10! =  3628800
6! =  720

Method 2: Using Iterative method

The factorial of a number can also be calculated using iterative method.

<?php
function factorial($x) {
  $finalnum = 1;
  for($i = $x; $i > 0; $i--) {
    $finalnum = $finalnum * $i;
  }
  return $finalnum;  
}

echo "10! =  ".factorial(10)."\n";
echo "6! =  ".factorial(6)."\n";
?>

The above code will give the following output:

10! =  3628800
6! =  720

Method 3: Using Ternary Operator

In the below example, the factorial of a number is calculated using ternary operator.

<?php
function factorial($x) {
  $y = ($x == 0 || $x == 1)? 1 : $x*factorial($x-1);
  return $y;  
}

echo "10! =  ".factorial(10)."\n";
echo "6! =  ".factorial(6)."\n";
?>

The above code will give the following output:

10! =  3628800
6! =  720




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