I'd be carefull. a well chosen collection of characters could easily do some damage:
$clientQuery = "SELECT * FROM Clients WHERE client='$client'";
imagine $client is
'; DROP TABLE Clients WHERE ''='
this will result in the query:
SELECT * FROM Clients WHERE client=''; DROP TABLE Clients WHERE ''=''
which could be very dangerous for your information. Either check for bad data, or block it using mysql_real_escape_string