php class do once for all instances - methods

Question

So I have a class whose constructor downloads some xml and reads it into properties for the class to work with. I'm instantiating this class a couple of times and this time-consuming job is done three times in exactly the same manner. Could I avoid it somehow (I guess with a static method/properties)? My class should only once go get the properties and then every instance could use them. I feel I should put the code out of my constructor in a static function, but how it's done exactly, I don't know, since I always get errors.
class MyClass {
protected $xml_file;
protected $xml_derived_array;
public function __construct($param1, $param2, $param3) {
//get xml_file and make xml_derived_array with it
//do some other stuff with parameters and properties such as $xml_derived_array
}
}
Should become something like: (but how should I call the static properties in my __construct and how should I set the properties in the static function?)
class MyClass {
protected static $xml_file;
protected static $xml_derived_array;
protected static function get_xml() {
//get xml_file and make xml_derived_array with it (?how exactly?)
}
public function __construct($param1, $param2, $param3) {
self::get_xml();
//do some other stuff with parameters and properties such as $xml_derived_array (?how exactly?)
}
}
Edit
This is how it is now working:
class MyClass {
protected static $xml_file;
protected static $xml_derived_array = array();
public function __construct($param1, $param2, $param3) {
if (!self::$xml_file) {
self::$xml_file = simplexml_load_file('xml_file.xml');
self::$xml_derived_array[0] = self::$xml_file->title;
}
echo self::$xml_derived_array[0].$param1;
}
}

Answer 1

You don't need a static method. Just check if the property has a value in constructor and if not get the xml file and process it.
class MyClass {
protected static $xml_file;
protected static $xml_derived_array = array();
public function __construct($param1, $param2, $param3) {
if (! count(self::$xml_derived_array)){
//get xml_file and make xml_derived_array with it
//do some other stuff with parameters and properties such as $xml_derived_array
}
}
}