$mult = $r['level'] * $r['level'];
$chance = min(($ir['crimexp']/$mult) * 50 + 1, 95);
if(rand(1,100) < $chance)
{
$gain = $r['level'] * 5;
Basically,
My Level: 2
Crime Exp: 50
So,
$mult = 2 * 2; //4
$chance = min((50/4) * 50 + 1, 95); //~626
So, if the random generated number is below ~626 they succeed (The random generated number would be between 1 and 100)