使用PHP在类中声明变量

Fairly new to classes in PHP, so bear with me.

class processRoutes
    {   
        //Next line works
        private $doc = "works as as string";
        //Next line does not work, "Parse error: syntax error, unexpected T_NEW"
        private $doc = new SimpleXMLElement('routingConfig.xml', null, true);
        private function getTablenames()
        {
            //do stuff  
        }
    }

I'm trying to ultimately utilize the SimpleXMLElement object within my class, among several private functions. What is the correct method for going about this, and why doesn't my current method work?

You need to do this in your constructor, as this can't be evaluated at this stage of script parsing. 'Simple' values, as strings, bools and numeric values will work though.

class processRoutes
{   
  //Next line works
  private $doc = "works as as string";

  private $doc;

  public function __construct()
  {
     $this->doc = new SimpleXMLElement('routingConfig.xml', null, true);
  }

  // ....
}

anytime you want to reference a class's variable, use the keyword $this

public function getTablenames()
{
    $my_new_variable = $this->doc; // Transfers the $doc variable
}

You're attempting to initialise a property with an object instance, but you're only allowed to initialise variables with constants that can be determined at "compile time".

From PHP Manual - Properties

This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

Any initialisation that depends on "run time" knowledge will need to be executed either

• in a constructor (refer @Dan-Lee's answer on how to implement this) that operates on $this->doc,
• from within eg a "initialiser" function called from your constructor (you might do this to keep the "initialisation" steps distinct from other "real work" that might be done from your constructor), or
• manually by your class's consumer. The consumer may operate
  • directly on the properties or your object (eg myProcessRoutes->doc = 'some other string'),
  • via a function call to your object eg myProcessRoutes.initialise_doc('some other string'), or
  • via a setter on your object - I'll let you research those, as I've not used these in PHP! ;-)

(Although it's arguable/philosophical if these approaches that occur later than instantiation/constructor are really initialisation).

The point of class constructors/destructors is to provide a "hook" by which the object instance can be initialised/disposed as required.

You might just need to create some specific new instances as per your example, in which case you don't need to accept any input to the constructor from the consumer.

Or, you might need to accept some values in order for your class to be set up properly. This is exactly what's happening in your example code above, when you're calling

private $doc = new SimpleXMLElement('routingConfig.xml', null, true);

(that is, you're passing the values of 'routingConfig.xml', null and true in to your new instance of SimpleXMLElement, so that this instance's constructor can initialise the instance using the values you passed to it, ready for use).