Suite vBulletin 4 upgrade failed due to undefined function gzcompress()

[محروم.كوم]

During the upgrade process to the latest vBulletin 4 at http://sitename/vbulletin/install/fi...ade.php?step=7, I saw the following:
Code:
Step 7) Merge template changes
Fatal error: Call to undefined function gzcompress() in

/usr/local/apache2/htdocs/vbulletin/includes/class_diff.php on line 170
The section of the file causing this problem is this:
Code:
/**
* Compresses a row of the LCS table in order to reduce memory usage of the table.
*
* @param array Array of entries in a row of the LCS table
*
* @return string A pipe delimited version of the row
*/
function compress_row($row)

{
return gzcompress(implode('|', $row), 9);
}
What I don't understand is how this happens, and what I should do to resolve this?

Also, during the previous step (ie Step 6), it asks for a Username for the CMS. I tried to enter the Admin user name for vBulletin and press the button next to it, rather than the button "Next" for the upgrade script. It gave me some error (can't remember what they're now). Just tried to change the browser's URL to step=6 and that section asking for the Username doesn't show up again. __DEFINE_LIKE_SHARE__