php. why function's running time is big?
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I am profiling my code in php. Question is about next function:

    // returns true if edge exists in the tree
    protected function edgeExist( $srcNodeId, $firstToken ) {
        $result = array_key_exists( $srcNodeId, $this->edges )
                  && array_key_exists( $firstToken, $this->edges[$srcNodeId]);
        return $result;
    }
According to the profiler, function `edgeExist` consumes about 10% of running time, but function `array_key_exists` consumes about 0.2% of running time.
Why does function `edgeExist` consume so much? 