If you want to send push notification on Android mobile please use the below function:

Requirement :: a) Mobile TokenId b) Google API Key

public function send_notification_mobile($mobiletokens, $GOOGLE_APIKEY){

             // API access key from Google API's Console
            define( 'API_ACCESS_KEY', $GOOGLE_APIKEY);
            $registrationIds = $mobiletokens;

            // prep the bundle
            $msg = array
            (
                'message'       => 'This is a testing message text',
                'title'         => 'This is a title. title',
                'subtitle'      => 'This is a subtitle. subtitle',
                'tickerText'    => 'Ticker text here...Ticker text here...',
                'vibrate'       => 1,
                'sound'         => 1,
            );

            $fields = array
            (
                'registration_ids'  => $registrationIds,
                'data'              => $msg,
                );

            $headers = array
            (
                'Authorization: key=' . API_ACCESS_KEY,
                'Content-Type: application/json'
            );

            $ch = curl_init();
            curl_setopt( $ch,CURLOPT_URL, 'https://android.googleapis.com/gcm/send' );
            curl_setopt( $ch,CURLOPT_POST, true );
            curl_setopt( $ch,CURLOPT_HTTPHEADER, $headers );
            curl_setopt( $ch,CURLOPT_RETURNTRANSFER, true );
            curl_setopt( $ch,CURLOPT_SSL_VERIFYPEER, false );
            curl_setopt( $ch,CURLOPT_POSTFIELDS, json_encode( $fields ) );
            $result = curl_exec($ch );
            curl_close( $ch );
            return $result;        
    }