Set comma after string write in mysql query in PHP

This is driving me nuts, I thought i had it but guess not.. I am simply trying to add a comma after the string write in this query:
 $sql = "UPDATE user SET session_requests='$usernumber' WHERE username ='$un'";

What is the proper way to add a comma to session_requests so that say the usernumber is 5 it will read "5,"

I've tried filling a variable with comma like
$comma = ',';

then doing this
$sql = "UPDATE user SET session_requests='$usernumber'.$comma WHERE username ='$un'";
but that doesn't work

Tried it about 4 other ways too. If you can help me out can you show me the ways to do this by:
1) assigning a comma to a variable
2) doing it without a variable like
$sql = "UPDATE user SET session_requests='$usernumber'.',' WHERE username ='$un'";
or whatever works..

Thanks a bunch!
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Guy Hengel [angelIII / a3]Billing EngineerCommented:
$sql = "UPDATE user SET session_requests='$usernumber,' WHERE username ='$un'";

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The main problem you are having is that you are putting the dot/concatenation operator (.) into your SQL statement, but that's the PHP syntax.  angelIII has given you the best solution and deserves all the points, but if for some crazy reason you did want to do the concatenation on the SQL side it would be:

$sql = "UPDATE user SET session_requests='$usernumber'+',' WHERE username ='$un'";

The only change being that you use a plus (+) instead of a dot (.).
stormistAuthor Commented:
Ok wow that was easy. Works now. AngelIII how would I do it if I wanted to put the comma in a variable? Oh and Thanks! :)

stormistAuthor Commented:
Thanks for the comments JoshdanG, I understand. Any idea how to put two variables next to each other?? use '+' as well? like
$sql = "UPDATE user SET session_requests='$usernumber'+'$comma'' WHERE username ='$un'";
Guy Hengel [angelIII / a3]Billing EngineerCommented:

$sql = "UPDATE user SET session_requests='$usernumber$comma' WHERE username ='$un'";
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