You can access property names with dashes in them (for example, because you converted an XML file to an object) in the following way:

<?php
$ref = new StdClass();
$ref->{'ref-type'} = 'Journal Article';
var_dump($ref);
?>

$this can be cast to array.  But when doing so, it prefixes the property names/new array keys with certain data depending on the property classification.  Public property names are not changed.  Protected properties are prefixed with a space-padded '*'.  Private properties are prefixed with the space-padded class name...

<?php

class test
{
    public $var1 = 1;
    protected $var2 = 2;
    private $var3 = 3;
    static $var4 = 4;
    
    public function toArray()
    {
        return (array) $this;
    }
}

$t = new test;
print_r($t->toArray());

/* outputs:

Array
(
    [var1] => 1
    [ * var2] => 2
    [ test var3] => 3
)

*/
?>

This is documented behavior when converting any object to an array (see </language.types.array.php#language.types.array.casting> PHP manual page).  All properties regardless of visibility will be shown when casting an object to array (with exceptions of a few built-in objects).

To get an array with all property names unaltered, use the 'get_object_vars($this)' function in any method within class scope to retrieve an array of all properties regardless of external visibility, or 'get_object_vars($object)' outside class scope to retrieve an array of only public properties (see: </function.get-object-vars.php> PHP manual page).

Do not confuse php's version of properties with properties in other languages (C++ for example).  In php, properties are the same as attributes, simple variables without functionality.  They should be called attributes, not properties.

Properties have implicit accessor and mutator functionality.  I've created an abstract class that allows implicit property functionality.

<?php

abstract class PropertyObject
{
  public function __get($name)
  {
    if (method_exists($this, ($method = 'get_'.$name)))
    {
      return $this->$method();
    }
    else return;
  }
  
  public function __isset($name)
  {
    if (method_exists($this, ($method = 'isset_'.$name)))
    {
      return $this->$method();
    }
    else return;
  }
  
  public function __set($name, $value)
  {
    if (method_exists($this, ($method = 'set_'.$name)))
    {
      $this->$method($value);
    }
  }
  
  public function __unset($name)
  {
    if (method_exists($this, ($method = 'unset_'.$name)))
    {
      $this->$method();
    }
  }
}

?>

after extending this class, you can create accessors and mutators that will be called automagically, using php's magic methods, when the corresponding property is accessed.

When declaring class properties in PHP, keep the following rules in mind:

A property must have a visibility modifier (public, protected, or private).
You may optionally use type declarations (available since PHP 7.4).
If you assign a default value, it must be a constant expression.

This means you can only assign fixed values such as integers, strings, booleans, or constants. You cannot use function calls or any runtime-evaluated expressions.

Example (valid):

class Test {
    public int $x = 10;
    public string $name = "Rahim";
    public bool $flag = true;
}

Example (invalid):

class Test {
    public int $x = rand(); // Fatal error: constant expression required
}

The reason is that property defaults are evaluated at compile time, while functions like rand() are executed at runtime.

If you need dynamic values, assign them inside the constructor or a method:

class Test {
    public int $x;

    public function __construct() {
        $this->x = rand(); // OK
    }
}

Note: This rule applies whether or not you use typed properties (PHP 7.4+).