What are some common mistakes that beginners make when working with classes and methods in PHP?

Common mistakes that beginners make when working with classes and methods in PHP include not properly defining methods within a class, forgetting to use the $this keyword to access class properties and methods, and not properly instantiating objects of a class before using its methods. To solve these issues, make sure to define methods within a class using the "function" keyword, use the $this keyword to access class properties and methods within the class definition, and instantiate objects of a class before calling its methods.

class MyClass {
    public $property;

    public function myMethod() {
        // method logic here
    }
}

$myObject = new MyClass();
$myObject->myMethod();