PHP OOP：所有类实例中的唯一ID属性

My question is simple, but I can't seem to find any answer for it online. I will probably jump straight into the code:

class NewClas {
    public $id;

    public function __construct($id) {
        $this->id = $id;

        $this->checkVars();
    }

    public function checkVars() {
        if (empty($this->id)) {
            trigger_error('ID is a required parameter.');
        } elseif ($this->id WAS USED IN A PREVIOUS OBJECT) {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }

}

$object1 = new NewClass('id1');
$object2 = new NewClass('id2');
$object3 = new NewClass('id1'); // throws error, because id1 was already used

So - is it possible to check for uniqueness of a value of the property among all instances of the class? I am just getting started with OOP, so please go easy on me. :)

Also, I am aware of spl_object_hash but I would prefer work with IDs as readable strings, specified by a user.

Thanks in advance!

It is possible - if you'll store static registry of used id's. That's about:

class NewClass
{
    public $id;
    //here's your registry
    protected static $registry = array();

    public function __construct($id) 
    {
        $this->id = $id;

        $this->checkVars();
        //if not failed, add to registry:
        self::$registry[] = $id;
    }

    public function checkVars() 
    {
        if (empty($this->id)) 
        {
            trigger_error('ID is a required parameter.');
        }
        //checking if it's already used: 
        elseif (in_array($this->id, self::$registry)) 
        {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }

}

You can check this demo

It won't throw any error. You are triggering the error using the trigger_error under the else block. That's the reason you are getting an error.

When you do this..

$object3 = new NewClass('id1');

The id1 is passed as the parameter to the constructor and it is set to the $id public variable. Now checkVars() is going to be called .. Here $this->id will not be empty, so it will go to the else block.

This is the right code actually..

<?php

class NewClass {
    public $id;

    public function __construct($id) {
        $this->id = $id;

        $this->checkVars();
    }

    public function checkVars() {
        if (empty($this->id)) {
            trigger_error('ID is a required parameter.');
        } else {
           // trigger_error('ID is already used.');
        }
    }

}

$object1 = new NewClass('id1');
$object2 = new NewClass('id2');
$object3 = new NewClass('id1'); 

This is the right answer from above answer: But to respect SOLID OOP design principles I would recommend to make id private and use getters and setters to access it.

class NewClass
{
    private $id;
    //here's your registry
    public static $registry = array(); //since is static you can make it public

    public function __construct($id) 
    {
        $this->id = $id;

        $this->checkVars();
        //if not failed, add to registry:
        self::$registry[] = $id;
    }

    public function checkVars() 
    {
        if (empty($this->id)) 
        {
            trigger_error('ID is a required parameter.');
        }
        //checking if it's already used: 
        else if (in_array($this->id, self::$registry)) 
        {
            trigger_error('ID "'.$this->id.'" was used already. Please insert a unique name.');
        }
    }