public static function backup($tables, $file)
{
    $return       = ''; // LINE 1221
    $file_written = false;

    //get all of the tables
    if ($tables == '*') {
        $tables = array(); // LINE 1226
        $result = mysqli_query(self::$cn, 'SHOW TABLES');
        while ($row = mysqli_fetch_row($result)) {
            $tables[] = $row[0];
        }
    } else {
        $tables = is_array($tables) ? $tables : explode(',', $tables);
    }

    // Initialize File
    $filename = $file . 'db-backup-' . time() . '-' . (md5(implode(',', $tables))) . '.sql';
    echo $filename; die(); // placed this here so I could see
                           // if it was running out of memory above
                           // and this echos out fine, so the problem
                           // is somewhere below.
    $handle   = fopen($filename, 'w');
    fclose($handle);
    $handle = fopen($filename, 'a');

    //cycle through
    foreach ($tables as $table) {
        if ($table <> "") {
            $result = mysqli_query(self::$cn, 'SELECT * FROM `' . $table . '`');
            if ($result) {
                $num_fields = mysqli_num_fields($result);

                $row2 = mysqli_fetch_row(mysqli_query(self::$cn, 'SHOW CREATE TABLE `' . $table . '`'));
                if ($row2) {
                    $file_written = true;
                    $return .= 'DROP TABLE IF EXISTS `' . $table . '`;';
                    $return .= "nn" . $row2[1] . ";nn";

                    $field_flags = false;
                    fwrite($handle, $return);
                    $return = '';

                    for ($i = 0; $i < $num_fields; $i++) {
                        while ($row = mysqli_fetch_row($result)) {

                            if ($field_flags == false) {
                                for ($j = 0; $j < $num_fields; $j++) {
                                    $tmp = mysqli_fetch_field_direct($result, $j);
                                    if (strpos($tmp->flags, 'NOT_NULL') !== false) {
                                        $field_flags[$j] = false;
                                    } else {
                                        $field_flags[$j] = true;
                                    }
                                }
                            }

                            // var_dump($field_flags); die();
                            $return .= 'INSERT INTO `' . $table . '` VALUES(';
                            for ($j = 0; $j < $num_fields; $j++) {
                                $row[$j] = addslashes($row[$j]);
                                $row[$j] = preg_replace("/n/", "\n", $row[$j]);
                                if (isset($row[$j])) {
                                    if ($row[$j] <> '') {
                                        $return .= '"' . $row[$j] . '"';
                                    } else {
                                        if ($field_flags[$j]) {
                                            $return .= "NULL";
                                        } else {
                                            $return .= '""';
                                        }
                                    }
                                } else {
                                    if ($field_flags[$j]) {
                                        $return .= "NULL";
                                    } else {
                                        $return .= '""';
                                    }
                                }
                                if ($j < ($num_fields - 1)) {
                                    $return .= ',';
                                }
                            }
                            $return .= ");n";
                            fwrite($handle, $return);
                            $return = '';
                        }
                    }
                    $return .= "nnn";
                    fwrite($handle, $return);
                    $return = '';
                }
            }
            mysqli_free_result($result);
        }
    }
    fclose($handle);

    //save file
    if ($file_written) {
        return $filename;
    } else {
        if (file_exists($filename)) {
            unlink($filename);
        }
        return false;
    }
}

PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 32 bytes) in d:serverhtdocsbackup.php on line 1221, referer: http://example.com/index.html
PHP Stack trace:, referer: http://example.com/index.html
PHP   1. {main}() d:serverhtdocsbackup.php:0, referer: http://example.com/index.html
PHP   2. mysqli_db::backup() d:serverhtdocsbackup.php:31, referer: http://example.com/index.html
PHP   3. mysqli_fetch_field_direct() d:serverhtdocsbackup.php:1221, referer: http://example.com/index.html
PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 32 bytes) in d:serverhtdocsbackup.php on line 1226, referer: http://example.com/index.html

$result = mysqli_query(self::$cn, 'SELECT * FROM `' . $table . '`', MYSQLI_USE_RESULT);