php - Fatal error when extending included class -

i trying simple thing here doesn't seem work. take @ code:

include 'custom/mainclass.php';  $child = new childclass();  class childclass extends mainclass { } 

apparently childclass() cannot found (according php).. i'm 100% sure i'm doing stupid in way of ordering code.

i searched web understand i'm doing nothing wrong..

you have declare classes in code first before using them.

include 'custom/mainclass.php';  class childclass extends mainclass {  }  $child = new childclass(); //create instance after class has been declared 

edit:

after research turned out, can use class before declaring it. but, declaration of class , parent classes must in same file.

so if declare parent class in 1 file , child class in another, won't work.

also, must declare parent classes first. after can extend them.