Consider the following code:

<?php
echo str_repeat("x", 4095);
header("Location: /somewhere_else.php");
exit;
?>

If zlib.output_compression is off, this will cause 4095 "x"s to display, properly followed by an error on the header line.

If zlib.output_compression is on, however, the echo line will execute - but nothing is output immediately because it fails to fill the compression buffer - so the header line goes ahead and executes and redirects successfully, completely spurning the output.

Granted, nobody would use this particular example in a real world application - I just set it up to illustrate the problem. I have run into real situations, though, where I'm doing diagnostics or inadvertently cause an error on code lines that logically precede a header("Location:... call. This problem prevents my diagnistic or error messages from showing up. Consider:

<?php
// set up mysql connection and
mysql_query("insert into mytable (myfield) values (". $_GET["myvalue"]. ")");

// no problem if $_GET["myvalue"] is a number,
// but if it's a non-numeric string like "abc" that line will generate an error message
// which I would like to see so I can fix the problem...

// but the error message doesn't fill the compression buffer
// so the following redirect happens and usurps the error message
header("Location: /somewhere_else.php");
exit;

// so I go on as though there was never a problem
// until I look at my still-empty database table some time later...
?>