Description:
------------
When running the following code, the output in browser isn't what one would expect (there's a "&" missing).
If one outputs something after the echo(substr()) (like, echo "test") it works fine, also if one outputs characters after the "?>"

This bug has been confirmed by Saveas on #php, freenode, running PHP 4.3.2 on Debian Apache1, and by me on 2 mod_php 4.3.9, Apache 2.0.51 Gentoo Linux systems (twice another configuration tough)

It does look like a buffer flush problem.

Reproduce code:
---------------
<?php
$t = array("t","e","s","t","j","e");
array_push($t, "t");
array_unshift($t, "");
var_dump($t);
echo "\n<br />";
$tmp = implode("&", $t);
echo $tmp;
echo "\n<br />";
echo(substr($tmp, 0, -1));
//echo "<br />test";
?>

Expected result:
----------------
array(8) { [0]=>  string(0) "" [1]=>  string(1) "t" [2]=>  string(1) "e" [3]=>  string(1) "s" [4]=>  string(1) "t" [5]=>  string(1) "j" [6]=>  string(1) "e" [7]=>  string(1) "t" }
&t&e&s&t&j&e&t
&t&e&s&t&j&e&

Actual result:
--------------
array(8) { [0]=>  string(0) "" [1]=>  string(1) "t" [2]=>  string(1) "e" [3]=>  string(1) "s" [4]=>  string(1) "t" [5]=>  string(1) "j" [6]=>  string(1) "e" [7]=>  string(1) "t" }
&t&e&s&t&j&e&t
&t&e&s&t&j&e