Check if a string is an email address in PHP

I am trying to do an SQL query, but I need to check somehow if the value is an email address. I need a way to check if $user is an email address, because I have user values such as this in my table.

test
test2
test@example.com
test2@example.com
test392
test@example.net

and so on...

I need to make it so $useremail checks $user to find if it's an email address. So I can UPDATE the values, WHERE user=test OR user=test@example.com, etc.

$user = strtolower($olduser);
$useremail = "";

mysql_query("UPDATE _$setprofile SET user=$sn, fc=$fc WHERE user='$user' OR user='$useremail");