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If a user tries to register or wants to confirm his mail-address, the user gets
the following error: is_a(): Deprecated. Please use the instanceof operator 
The mail although was send successfully.

File: /includes/StubObject.php
Line: 27

return is_object( $obj ) && !is_a( $obj, 'StubObject' );

My solution:
return is_object( $obj ) && !($obj instanceof StubObject);