PHP property doesn't exist but no errors thrown when used

Question

So I came across a code like this and it makes use of one of the Bar class properties called testObj while it's not defined so I expected this to be wrong but I tested it my own and no errors:

    <?php
class Foo{
    public function __construct()
    {
        echo 'Echo From Foo';
    }
}
class Bar{
    public function __construct(Foo $foo)
    {
        $this->testObj = $foo;
    }
}

$bar = new Bar(new Foo);

Why is that so? Does this have anything to do with the "Dynamic/Loose Type"ness of PHP or something else?

Answer 1

Properties can be defined dynamically and their visibility is defaulted to public as can be seen in the example below:

class X {
  public function test()
  {
    $this->y = 'test';
  }
}

$x = new X();

$x->test();

echo $x->y; // test

You can also do this without being in the class, so if I wanted to add another property, I could just do the following:

class X {
  public function test()
  {
    $this->y = 'test';
  }
}

$x = new X();

$x->test();

echo $x->y; // test

$x->z = 'blah';

echo $x->z; // blah

Remember, when a class in instantized it is just an object which can be manipulated as any other object.

Note: If I don't call test() in the above code, it will result it an error (undefined property) because the variable has not been defined except in the test() function.

Live Example

Repl

Answer 2

This is perfectly normal, you can dynamicaly assign every propteries to a PHP class without error. Event if you should declare it properly to keep track of your object structure. If you want it to throw an error, you can use the __get magic function and ReflectionClass to determine wich property is setted and wich you can't set even if I didn't see any advantage of doing this.