PHP Warning included in mysql.gz file breaking the archive.

I did the backup from UI and then downloaded the file and it seems it's invalid.
The mysql.gz file starts from:

<br />
<b>Warning</b>:  Illegal string offset 'value' in <b>backup_migrate/includes/files.inc</b> on line <b>340</b><br />
<br />
<b>Warning</b>:  Illegal string offset 'value' in <b>backup_migrate/includes/files.inc</b> on line <b>340</b><br />
<br />
<b>Warning</b>:  Illegal string offset 'key' in <b>backup_migrate/includes/files.inc</b> on line <b>341</b><br />
<br />
<b>Warning</b>:  Illegal string offset 'value' in <b>backup_migrate/includes/files.inc</b> on line <b>341</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at backup_migrate/includes/files.inc:340) in <b>includes/bootstrap.inc</b> on line <b>1221</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at backup_migrate/includes/files.inc:340) in <b>includes/bootstrap.inc</b> on line <b>1221</b><br />
^_<8B>^H^@^@^@^@^@^B^C<EC><BD><DD>r<E3>8<92>0z<BD><F3>^T\

This make the archive in invalid format.

Failing code:

        // To prevent HTTP header injection, we delete new lines that are
        // not followed by a space or a tab.
        // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2
        $header['value'] = preg_replace('/\r?\n(?!\t| )/', '', $header['value']);