simple math question

[esoteric]

What is the correct way to right a simple maths question

 

i have my total as:

<?php echo $row['total'];?>

 

i have my another number as:

<?php echo $row['user_credit']; ?>

 

but what i want to do is subtract the user_credit from the total and save it as a new variable called 'credits_used' but im unsure how to write it

 

do i do it like

$row['total'] - $row['user_credit'] = $row['credit_used'];

 

Thank for any help

[xyph]

$row['credit_used'] = $row['user_credit'] - $row['total'];