<?php
echo substr("kesavan",1)."<br />";
echo substr("kesavan", 1,3)."<br />";
echo substr("kesavan",-1)."<br />";
echo substr("kesavan", -2."<br />";
echo substr("kesavan", 1, -2)."<br />";
echo substr("kesavan",-5,-2)."<br />";
?>

esavan
esa
n
an
esav
sav

PHP's substr() function returns a part of a string based on the parameters supplied to it. For example:

1. substr("kesavan",1) would return "esavan" (the 1 here specifies the specifies the position of the first character of the substring [0th position->k, 1st position->e, 2nd position->s and so on in the string "kesavan"]).
   
2. substr("kesavan", 1,3) would return "esa", yes you got it right, the 3rd parameter specified inside substr() ("3" in this case) is the length of the substring.
   
3. substr("kesavan",-1) would return "n", so if the 2nd parameter is negative the substring first character is got by looking at the string from it's end.
   
4. substr("kesavan", -2) would return "an" and so on.
5. Another case to be considered is when the 3rd parameter (length of the substring) is negative. In that case that many characters are to be omitted from the main string to get the substring.
   substr("kesavan", 1, -2) would return "esav", thus the last two characters in the main string are dropped to get the substing.
   
6. Now what if the 2nd parameter (start of the substring) and the third character(length of the substring) are both negative?
   substr("kesavan",-5,-2), now this would return "sav" (dropping the last two characters in the main string).
   

More info can be found here.