What OOP construct unavailable in PHP 4 does the following script approximate?

class my_class
{
function my_funct ($my_param)
{
user_error ("Please define me", E_ERROR);
}
function b()
{
return 10;
}
}

A. Multiple inheritance
B. Interfaces
C. Abstract methods
D. Private methods
E. Function overloading

Answer:
Abstract methods