http://www.mantisbugtracker.com/requirements.php states PHP 4.0.6 and higher.

However, with PHP 4.2.2, the installation script is unable to create config_inc.php because the fopen() call uses the 'x' flag which wasn't introduced until PHP 4.3.2.

Propose changing the flag to 'w'. The fopen() call is already guarded by a file existence check, e.g.,

if ( !file_exists ( $t_config_filename ) ) {
if ( $fd = @fopen( $t_config_filename, 'w' ) ) {
fwrite( $fd, $t_config );
fclose( $fd );
}