[PHP] Queries - Sometimes I need to use apostrophe, other times idon't

[Phil Schwarzmann]

When running a mysql_query, sometimes i need to have apostrophes
surrounding variables...other times i don't
 
example...
 
$query = "SELECT * FROM table WHERE id='$id'";
 
or
 
$query = "SELECT * FROM table WHERE id=$id";
 
...it kinda seems random when it wants apostrophes and when it doesn't
want them.
 
Any ideas?