backup up db using php script and cron job

Question

I'm using the script I posted below and it appeared to have worked based on the email and the fact it actually generated the file. However, It didn't backup anything.

Here is the output stored in the file it created:

SET FOREIGN_KEY_CHECKS=0;

SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";

SET AUTOCOMMIT=0;

START TRANSACTION;

SET FOREIGN_KEY_CHECKS=1;

COMMIT;

Here is the php script being used to generate the backup:

    #!/usr/bin/php
<?php

backup_tables();

// backup all tables in db
function backup_tables()
{
    $day_of_backup = ''; //possible values: `Monday` `Tuesday` `Wednesday` `Thursday` `Friday` `Saturday` `Sunday`
    $backup_path = '/home/user/public_html/path/to/BACKUP.file/destination/'; //make sure it ends with "/"
    $db_host = 'localhost';
    $db_user = 'root';
    $db_pass = 'replace_w_ur_password';
    $db_name = 'replace_w_ur_db_name';

    //set the correct date for filename
    if (date('l') == $day_of_backup) {
        $date = date("Y-m-d");
    } else {
        //set $date to the date when last backup had to occur
        $datetime1 = date_create($day_of_backup);
        $date = date("Y-m-d", strtotime($day_of_backup.'-1 days'));
    }

    if (!file_exists($backup_path.$date.'-backup'.'.sql')) {

        //connect to db
        $link = mysqli_connect($db_host,$db_user,$db_pass);
        mysqli_set_charset($link,'utf8');
        mysqli_select_db($link,$db_name);

        //get all of the tables
        $tables = array();
        $result = mysqli_query($link, 'SHOW TABLES');
        while($row = mysqli_fetch_row($result))
        {
            $tables[] = $row[0];
        }

        //disable foreign keys (to avoid errors)
        $return = 'SET FOREIGN_KEY_CHECKS=0;' . "\r\n";
        $return.= 'SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";' . "\r\n";
        $return.= 'SET AUTOCOMMIT=0;' . "\r\n";
        $return.= 'START TRANSACTION;' . "\r\n";

        //cycle through
        foreach($tables as $table)
        {
            $result = mysqli_query($link, 'SELECT * FROM '.$table);
            $num_fields = mysqli_num_fields($result);
            $num_rows = mysqli_num_rows($result);
            $i_row = 0;

            //$return.= 'DROP TABLE '.$table.';'; 
            $row2 = mysqli_fetch_row(mysqli_query($link,'SHOW CREATE TABLE '.$table));
            $return.= "\n\n".$row2[1].";\n\n"; 

            if ($num_rows !== 0) {
                $row3 = mysqli_fetch_fields($result);
                $return.= 'INSERT INTO '.$table.'( ';
                foreach ($row3 as $th) 
                { 
                    $return.= '`'.$th->name.'`, '; 
                }
                $return = substr($return, 0, -2);
                $return.= ' ) VALUES';

                for ($i = 0; $i < $num_fields; $i++) 
                {
                    while($row = mysqli_fetch_row($result))
                    {
                        $return.="\n(";
                        for($j=0; $j<$num_fields; $j++) 
                        {
                            $row[$j] = addslashes($row[$j]);
                            $row[$j] = preg_replace("#\n#","\\n",$row[$j]);
                            if (isset($row[$j])) { $return.= '"'.$row[$j].'"' ; } else { $return.= '""'; }
                            if ($j<($num_fields-1)) { $return.= ','; }
                        }
                        if (++$i_row == $num_rows) {
                            $return.= ");"; // last row
                        } else {
                            $return.= "),"; // not last row
                        }   
                    }
                }
            }
            $return.="\n\n\n";
        }

        // enable foreign keys
        $return .= 'SET FOREIGN_KEY_CHECKS=1;' . "\r\n";
        $return.= 'COMMIT;';

        //set file path
        if (!is_dir($backup_path)) {
            mkdir($backup_path, 0755, true);
        }

        //delete old file
        //$old_date = date("Y-m-d", strtotime('-4 weeks', strtotime($date)));
        //$old_file = $backup_path.$old_date.'-backup'.'.sql';
        //if (file_exists($old_file)) unlink($old_file);

        //save file
        $handle = fopen($backup_path.$date.'-backup'.'.sql','w+');
        fwrite($handle,$return);
        fclose($handle);
    }
}

?>

Any recommendations or leads on why it wouldn't actually backup all the tables would be greatly appreciated.