how do i do the calculation with a variable in PHP?

Ok so i have 3 variables

 $real_price = 33.00;
 $real_option = 13.00;
 $option_value['price_pre开发者_如何学运维fix'] = "-";
 $real_total = $real_price $option_value['price_prefix'] $real_option; 

how come i cant do line 4

I need $real_total to be 20.00

i get Parse error: syntax error, unexpected T_VARIABLE

 $real_price = 33.00;
 $real_option = 13.00;
 $option_value['price_prefix'] = "-";
 eval('$real_total = $real_price '.$option_value['price_prefix'].' $real_option'); 

Althrough I don't recommend doing it like that. Do it like this:

 $real_price = 33.00;
 $real_option = 13.00;
 $option_value['price_prefix'] = "-";
switch($option_value['price_prefix']) {
case '+': $real_total=$real_price+$real_option; break;
case '-': $real_total=$real_price-$real_option; break;
case '*': $real_total=$real_price*$real_option; break;
}