Generally because of "bad programming", and a possibility for mistakes now or later.

1. If it's a mistake, make a proper assignment to the variable first: $varname=0;
2. If it really is only defined sometimes, test for it: if (isset($varname)) .... before using it
3. If it's because you spelled it wrong, just correct that
4. Maybe even turn of the warnings in you PHP-settings

Generally because of "bad programming", and a possibility for mistakes now or later.

1. If it's a mistake, make a proper assignment to the variable first: $varname=0;
2. If it really is only defined sometimes, test for it: if (isset($varname)), before using it
3. If it's because you spelled it wrong, just correct that
4. Maybe even turn of the warnings in you PHP-settings