RosettaCodeData/Task/Ackermann-function/PHP/ackermann-function.php

function ackermann( $m , $n )
{
    if ( $m==0 )
    {
        return $n + 1;
    }
    elseif ( $n==0 )
    {
        return ackermann( $m-1 , 1 );
    }
    return ackermann( $m-1, ackermann( $m , $n-1 ) );
}

echo ackermann( 3, 4 );
// prints 125