add ` to SQL query

Hello,
Should I put my varible between ` notes, link: `id` ?
in this example:

$query = mysql_query("SELECT id FROM `users` WHERE (first = '$first') AND (`email` = '$email')"); 

PHP:

is there any different if I use ` or not?

 

mysql recognizes backticks(`) as field name and single quote creates strings, so if you use backticks then you can use mysql keyword as field name too.

 

so in my example - should I use backticks like in EMAIL or withour backticks like in FIRST ?

 

you can use with or without the backticks unless it is a MySQL reserved keywords, e.g if you have a field name called select or from in your table you need to use backticks.
It will be better if you use backticks so you can separate field name and strings