Personally, I've been taught to always declare variables - whether I need to or not - I find it makes my code a lot easier to debug...

Also, you could use Hungarian notation (I think that's what this is) for naming variables - then you always know what a variable is meant to be used for:

string $sObjectName;
int $iNumObjects;
bool $bFinished;
float $fValue;

and so on....

Then if you find yourself doing:

$iObject = "myObject";

You'll know you're doing something wrong, as you can see at a quick glance that $iObject is meant to hold an int....