$basic = 500;
$fetch[amount]= "($basic * 10 / 100)";
$fetch[advance]="($basic * 10 / 100) + ($advance * 5 / 100) ";
$advance = str_replace('$basic', $basic, $fetch['amount']);
$advance= calculate_string($advance); // and is 50
echo $x = str_replace('$advance', $advance, $fetch['advance']);
// echo result (500 * 10 / 100) + ( * 5 / 100)
$advance is missing