SQL Injection

[Emanuele Zattin <emanuelez () mymachine mydomain com>]

Hello Everybody!
I recently found out that one of my websites suffered SQL injections like 
this:

Login: a' OR 'a'='a
Password: a' OR 'a'='a

I solved the problem checking whether the logon or password variables 
contained the "'" char... is it safe enough? i checked around the net and 
found a recent paper from Imperva but it does not talk about single chars 
checking... i tried to ude different encodings but that string in UTF-8 is 
just the same... any hint?