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GCD stands for Greatest Common Divisor. The GCD of two numbers is the largest number that divides both of them.

For example - GCD of 20 and 25 is 5, and GCD of 50 and 100 is 50.

Method 1: Using For Loop to find GCD of two numbers

In the below example, for loop is used to iterate the variable i from 0 to the smaller number. If both numbers are divisible by i, then it modifies the GCD and finally gives the GCD of two numbers.

<?php
$x = 50;
$y = 100;

if ($x > $y) {
  $temp = $x;
  $x = $y;
  $y = $temp;
}

for($i = 1; $i < ($x+1); $i++) {
  if ($x%$i == 0 and $y%$i == 0)
    $gcd = $i;
}

echo "GCD of $x and $y is: $gcd";
?>

The above code will give the following output:

GCD of 50 and 100 is: 50

Method 2: Using While Loop to find GCD of two numbers

In the below example, larger number is replaced by a number which is calculated by subtracting the smaller number from the larger number. The process is continued until the two numbers become equal which will be GCD of two numbers.

<?php
$p = $x = 20;
$q = $y = 25;

while ($x != $y) {
  if ($x > $y)
    $x = $x - $y;
  else
    $y = $y - $x;
}

echo "GCD of $p and $q is: $x";
?>

The above code will give the following output:

GCD of 20 and 25 is: 5

Method 3: Using the recursive function to find GCD of two numbers

In the below example, recursive function is used. In this method, instead of using subtraction operator(as in above example), modulus operator is used. This method is also known as Euclidean algorithm.

<?php
function gcd($x, $y) {
  if ($y == 0)
    return $x;
  return gcd($y, $x%$y);
}

$x = 250;
$y = 475;

echo "GCD of $x and $y is: ".gcd($x,$y);
?>

The above code will give the following output:

GCD of 250 and 475 is: 25