What types of methods are allowed when declared inside an abstract class?

In an abstract class in PHP, the following types of methods are allowed:

• Abstract methods — declared without implementation and must be implemented in child classes. Declared with the abstract keyword.

• Regular methods with implementation — can contain specific code that is inherited by child classes.

• Static methods — methods that can be called without creating an instance of the class.

• Methods with different visibility scopes — public, protected, private.

Example:

abstract class Animal {
    abstract public function makeSound(); // abstract method

    public function sleep() {
        echo "Sleeps...";
    }

    protected static function helper() {
        // static protected method
    }
}

Thus, an abstract class can contain both abstract methods and methods with implementation, allowing to define a common interface and partially implement functionality.