静态方法PHP-OOP

I want to access Static method properties to other static method. I am trying to access it via:

public static function getGender($user) {   
    $user_data[] = Common::getSomeUsers(true);
    $ssn_to_gender[] = array();
    foreach($user_data as $u_data){
        foreach($u_data as $key => $user){
            '$key:' .$user['ssn'];
            $ssn = $user['ssn'] . "<br/>";
            //echo $ssn;
            $ssn_gender= substr($ssn,7,-6) . "<br/>" ;
            $dob_gender= substr($ssn,0,6) . "<br/>" ;
            echo $dob_gender;
            if($ssn_gender % 2 == 0 )
            {
                echo 'F' . '</br>';
                $ssn_to_gender[$user['ssn']] = 'F';
            }
            elseif($ssn_gender % 2 == 1)
            {
                echo 'M' .'<br/>';
                $ssn_to_gender[$user['ssn']] = 'M';
            }

        }

    }
    return $ssn_to_gender;
    return $dob_gender;
    //$user= array();
    //echo Common::getSomeUsers($user);

}

public static function getAge( $user ) {


    self::getGender(true);
    Common::$dob_gender;

}
Common::getAge($userid='');
echo Common::$dob_gender;

I am trying to access $dob_gender variable in get_Age() method from getGender() method.

But unable to get the $dob_gender. Please guide I am new with PHP and object oriented programming.

Thanks.

I would think about doing a standard method instead of a series of static methods:

class Common
{
    private   $ssn_to_gender,
              $dob_gender;
    /**
    *  @returns  Itself (Common object)
    */
    public function getGender($user)
    {
        $user_data[]           =  $this->getSomeUsers(true);
        $this->ssn_to_gender[] =  array();

        foreach($user_data as $u_data){
            foreach($u_data as $key => $user){
                $ssn              =  $user['ssn'];
                $ssn_gender       =  substr($ssn,7,-6);
                $this->dob_gender = substr($ssn,0,6);

                if($ssn_gender % 2 == 0 ) {
                    $this->ssn_to_gender[$user['ssn']] = 'F';
                }
                elseif($ssn_gender % 2 == 1) {
                    $this->ssn_to_gender[$user['ssn']] = 'M';
                }
            }
        }

        return $this;
    }
    /**
    *  @returns  The corresponding private variable
    */
    public function getSSNToGender()
    {
        return $this->ssn_to_gender;
    }
    /**
    *  @returns  The corresponding private variable
    */
    public function getDOBGender()
    {
        return $this->dob_gender;
    }
    /**
    *  @question   I am not sure the purpose of this method. Presumably you need to
    *              do more in the way of script otherwise you would just do the
    *              getGender() method instead
    */
    public function getAge($userid)
    {
        # I am returning this but presumably you want to do something else??
        return $this->getGender($userid);
    }
}

To use would be like:

$Common = new Common();
$Age    = $Common->getAge($userid);

print_r($Age->getDOBGender());
print_r($Age->getSSNToGender());