Simplify a fraction

When you simplify a fraction, you divide the numerator and denominator by their greatest common divisor.

So all you need is to calcuate the GCD of the two numbers. There's no built-in function for that, but it's easy enough to implement the euclidean algorithm:

function gcd($a,$b) {
    $a = abs($a); $b = abs($b);
    if( $a < $b) list($b,$a) = Array($a,$b);
    if( $b == 0) return $a;
    $r = $a % $b;
    while($r > 0) {
        $a = $b;
        $b = $r;
        $r = $a % $b;
    }
    return $b;
}

Then just divide the top and bottom by that.

function simplify($num,$den) {
    $g = gcd($num,$den);
    return Array($num/$g,$den/$g);
}
var_export(simplify(40,100)); // Array(2,5)

If you have PHP gmp extension, you can do this.

$num = 40;
$den = 100;
$gcd = gmp_intval(gmp_gcd((string)$num, (string)$den));

$new_num = $num / $gcd;
$new_den = $den / $gcd;