Practical PHP Programming:Variable naming
From IpbWiki
Use meaningful variable names where appropriate. By "meaningful variables", I mean choose variable names that represent the data held inside, e.g. $member_name is likely to hold the name of a member, whereas $m_nm could hold anything. You should consider using underscores to separate words in your variables, because $membername is harder to read than $member_name - this is not required, though. Remember that variables need to be easy to type, so as to avoid the chance of being spelt incorrectly.
Use of variables such as $i, $j, $k, etc, are fine when used as temporary variables inside loops, but elsewhere are just meaningless and may cause confusion.
Good variable names: $date_of_birth, $firstname
Average variable names: $dob, $f_name, $first_name_of_current_member;
Bad variable names: $d, $fn, $n1