substr函数如何工作？ [关闭]

Question

I have come across this code in php.net.

<?php
echo substr('abcdef', 1);     // bcdef
echo substr('abcdef', 1, 3);  // bcd
echo substr('abcdef', 0, 4);  // abcd
echo substr('abcdef', 0, 8);  // abcdef
echo substr('abcdef', -1, 1); // f

// Accessing single characters in a string
// can also be achieved using "square brackets"
$string = 'abcdef';
echo $string[0];                 // a
echo $string[3];                 // d
echo $string[strlen($string)-1]; // f

?>

substr('abcdef',1,3) returns 'bcd', the last index is 3 and it returns "bcd" and if I do substr('abcdef',0,4) returns 'abcd'. But I expected substr('abcdef',0,3) to return 'abcd' as compared to the first example. What am I missing here.

How substr($string,$start,$last) works in php?

EDIT: Sorry every body I thought 3rd param is not length and something beyond my understanding...due to my Java background sorry. Thanks for all comments and answers.

Answer 1

substr() is NOT substr($string, $start, $last) it's substr($string, $start, $length). So substr('abcdef', 0, 4) returns abcd because it starts at 0 (a) and it goes for 4 letters.

Also random note: echo substr('abcdef', -1, 1); is inherently redundant since -1 only has a length of 1 so the string might as well be: echo substr('abcdef', -1);