Defines an error handler which is used in the test.
int $error_level: The level of the error raised.
string $message: The error message.
string $filename: The filename that the error was raised in.
int $line: The line number the error was raised at.
array $context: An array that points to the active symbol table at the point the error occurred.
Because this is registered in set_error_handler(), it has to be public.
public function customErrorHandler($error_level, $message, $filename, $line, $context) {
$this->lastErrorMessage = $message;
}