If you pass substr() a negative number, it counts from the end of the string.

 
<?php
$test = "this is a test.";
if ( $test = substr( $test, -3 ) == "est") {
  print "est";
} else {
  print "Welcome to our shop!";
}
?>

1.	A negative substr() length parameter
2.	A positive substr() length parameter
3.	Accessing Substrings: string substr ( string string, int start [, int length] )
4.	Extracting Part of a String with substr()
5.	Extracting a substring with substr()
6.	Extracting the end of a string with substr()
7.	Setting up and working with both strings and substrings
8.	Single-character substitutions
9.	Truncating a string with substr()
10.	Using substr() with length past the end of the string
11.	Using substr() with negative length
12.	Using substr() with negative start
13.	Using substr() with positive $start and $length
14.	Using substr() with positive start and no length
15.	string substr ( string str, int start_pos [, int length] ) reads part of a string and takes a minimum of two parameters
16.	substr() function returns the part of the string between the start and start+length parameters.
17.	substr-2.php
18.	substr-3.php
19.	substr.php
20.	Specify a negative number as parameter three for the length
21.	Using negative lengths allows you to say "copy everything but the last three characters,"
22.	Reading Fixed-Width Delimited Data

If you pass substr() a negative number, it counts from the end of the string. : substr « String « PHP