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"define a variable before the script runs"
-> how is that done? ;)

Here's a good test script for this.  Note that a 'undefined variable' is an error of level E_NOTICE, showing these errors depends on the error_reporting directive.

error_reporting(E_ALL); // turn our error_reporting up
print $undefined;       // error (notice)
$defined = 1;
print $defined;         // no error (it's set)