PHP and SQL maths -

this part of php , sql totally new me need use maths update table.

i have table avis_credits 1 row in table

the table deducts credits user uses system qty_credits, want able add additional credits account per example:

client has 10 credits left purchases 100 credits system must take current credits , add 100 giving me 110 credits. doing manual adding side , updating table expresion.

 "update avis_credits set credits='$qty_credit'"; 

what system looks current credits , adds new credits row.

i have tried

 "update avis_credits set credits='<?php echo qty_credit?> + $qty_credit'"; 

but not working @ all. because trying many functions in 1 string? or because maths equation wrong have no idea how write it?

on first query you're not doing math purely changing value of variable:

"update avis_credits set credits='$qty_credit'"; 

so credits becomes value of $qty_credit.

on second query <?php echo qty_credit?> wrong, you're trying open php tag on open 1 , don't use $ on variable , try sum variable gives error.

"update avis_credits set credits='<?php echo qty_credit?> + $qty_credit'"; 

this sanitize input , sum value of credits addition of value of variable $qty_credit:

$sql = sprintf("update avis_credits set credits = credits + '%s'",                 mysql_real_escape_string($qty_credit)); 

as side note, may want specify id of account want deposit additional credit otherwise above rule increase credit rows.

$sql = sprintf("update avis_credits set credits = credits + '%s' id = '%s'",                 mysql_real_escape_string($qty_credit),                mysql_real_escape_string($id));