I am saving operators(+,-,*) in mysql table have type varchar i have problem when in puting the values in formul e.g

$val1=7;
$val2=+;
$val3=5;
$val4=-;
$val5=9;

$ans=$val1.$val2.$val3.$val4.$val5;

the output is:

7+5-9

but i want to perform the action and give me the result 3

how to do?