"PHP Fatal error: Allowed memory size of 134217728 bytes exhausted" while generating database backup

Question 1

And one more: generation hungs at a large table (containing a log), but I think this is no matter 'cause information written row by row.

Actually this is the cause: I should use mysql_unbuffered_query instead mysql_query. Now this function looks like this:

function backup_tables($backup_filename, $tables = '*')
{
    $conf = new JConfig();
    $dbhost = $conf->host;
    $dbuser = $conf->user;
    $dbpassword = $conf->password;
    $dbname = $conf->db;
    $link = mysql_connect($dbhost, $dbuser, $dbpassword);
    mysql_select_db($dbname, $link) or die(mysql_error());
    $return = "drop database if exists `$dbname`;\n\ncreate database `$dbname`;\n\nuse `$dbname`;\n\n";
    $return .= "/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;\n\n";
    $return .= "/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;\n\n";
    $return .= "/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;\n\n";
    $return .= "/*!40101 SET NAMES utf8 */;\n\n";

    $handle = fopen($backup_filename, 'w+');
    fwrite($handle, $return); $return = "";

    // get all of the tables
    if ($tables == '*') {
        $tables = array();
        $result = mysql_query("SHOW TABLES");
        while ($row = mysql_fetch_row($result)) {
            $tables[] = $row[0];
        }
    } else {
        $tables = is_array($tables) ? $tables : explode(',', $tables);
    }

    // cycle through
    foreach ($tables as $table) {
        $return .= "DROP TABLE IF EXISTS `$table`;";
        $return .= "\n\n" . mysql_fetch_row(mysql_query("SHOW CREATE TABLE `$table`;"))[1] . " DEFAULT CHARSET=cp1251;\n\n";

        $result = mysql_unbuffered_query("SELECT * FROM `$table`");
        $num_fields = mysql_num_fields($result);

        while ($row = mysql_fetch_row($result)) {
            $return .= "INSERT INTO `$table` VALUES(";
            for ($i = 0; $i < $num_fields; $i++) {
                $row[$i] = str_replace("\n", "\\n", addslashes($row[$i]));
                $return .= '"' . (isset($row[$i])? $row[$i] : '') . '"';
                if ($num_fields - $i - 1) {
                    $return .= ',';
                }
            }
            $return .= ");\n";

            fwrite($handle, $return); $return = "";
        }
        if($return)
            fwrite($handle, $return);

        $return = "\n\n\n";
    }

    fclose($handle);
}

Question 2

The PHP answer here is to increase your max memory size if not your max execution time at the same time.

Outside of this being an exercise in re-creating the mysqldump command, is there a reason to perform this from within PHP code?

You might be better off using mysqldump or something like Holland http://hollandbackup.org/ to go through and dump each table individually.

Question 3

Current answer uses a deprecated function. The new way to do this is using mysqli::use_result.

In my case I ran into the exhausted memory error trying to write a large sql table into a file. Here's how I used it.

$conn = new mysqli("localhost", "my_user", "my_password", "my_db");

$sql = 'SELECT row1, row2 from table';

$fp = fopen('output.json', 'w');
if ($conn->multi_query($sql)) {
    do {
        if ($result = $conn->use_result()) {
            while ($row = $result->fetch_row()) {
              $row1 = $row[0];
              $row2 = $row[1];
              $item = array('row1'=>$row1, 'row2'=>$row2);  
              fwrite($fp, json_encode($item));
            }
            $result->close();
        }
    } while ($conn->more_results() && $conn->next_result());
}

fclose($fp);
$conn->close();