Update 2 records in the same time

this code:
$sql = "UPDATE members SET id = id + 1 WHERE id = '4'";

to Update one record(id)

but I want to update 2 records in the same time I use this code but it does not work:

$sql = "UPDATE members SET id = id + 1,UserName=$username WHERE id = '4'";

what is the error in this code ?

PHP Code:

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$sql = "UPDATE members SET id = '$id + 1',UserName='$username' WHERE id = '4'"; 


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id = '$id + 1': You need to get the id and store it in a variable, $id. You can't just add like that. You also need to put singe quotes (') around the values, ie: "UserName='$username'"