(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)
gmp_gcd — Calculate GCD
   Calculate greatest common divisor of num1 and
   num2. The result is always positive even if
   either of, or both, input operands are negative.
  
   A positive GMP number that divides into both
   num1 and num2.
  
Example #1 gmp_gcd() example
<?php
$gcd = gmp_gcd("12", "21");
echo gmp_strval($gcd) . "\n";
?>
The above example will output:
3