Spliting string based on length from end in PHP -

I want to split a string from the back. Like:

  $ mystring = "This is my string"; $ Mysecondstring = "histimistration";   
 I want to divide the last 6 characters from the above stars, here it is "string". How can I do PHP? By using  str_split , I can divide by the front, but I need it from the end.  
 Thanks in advance.   
 
 
 Use  
  edit   
  $ snippet = substr ($ mystring, 0), -6);