Create Backup.php file and add this code
<?php
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = '';
$dbname = 'database user name';
$backup_response = backup_Database($dbhost,$dbuser,$dbpass,$dbname);
  if($backup_response) {
  echo 'Database Backup Successfully Created!';  
  }
  else {
  echo 'Errors in Database Backup Creating!';    
  }
 


?>
<?php

function backup_Database($hostName,$userName,$password,$DbName,$tables = '*')
{
  
  // CONNECT TO THE DATABASE
  $con = mysql_connect($hostName,$userName,$password) or die(mysql_error());
  mysql_select_db($DbName,$con) or die(mysql_error());
  
  
  // GET ALL 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);
  }
  
  $return = 'SET FOREIGN_KEY_CHECKS=0;' . "rn";
  $return.= 'SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";' . "rn";
  $return.= 'SET AUTOCOMMIT=0;' . "rn";
  $return.= 'START TRANSACTION;' . "rn";
  
  
  foreach($tables as $table)
  {
    $result = mysql_query('SELECT * FROM '.$table) or die(mysql_error());
    $num_fields = mysql_num_fields($result) or die(mysql_error());
    
    $data.= 'DROP TABLE IF EXISTS '.$table.';';
    $row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
    $data.= "nn".$row2[1].";nn";
    
    for ($i = 0; $i<$num_fields; $i++) 
    {
      while($row = mysql_fetch_row($result))
      {
        $data.= 'INSERT INTO '.$table.' VALUES(';
        for($x=0; $x<$num_fields; $x++) 
        {
          $row[$x] = addslashes($row[$x]);
      $row[$x] = clean($row[$x]); // CLEAN QUERIES
          if (isset($row[$x])) { 
      	$data.= '"'.$row[$x].'"' ; 
      } else { 
      	$data.= '""'; 
      }
      
          if ($x<($num_fields-1)) { 
      	$data.= ','; 
      }
        }  // end of the for loop 2
        $data.= ");n";
      } // end of the while loop 
    } // end of the for loop 1
  
    $data.="nnn";
  }  // end of the foreach*/
  
    $return .= 'SET FOREIGN_KEY_CHECKS=1;' . "rn";
  $return.= 'COMMIT;';
  
  //SAVE THE BACKUP AS SQL FILE
  $f_name=$DbName.'-Database-Backup-'.date('Y-m-d @ h-i-s').'.sql';
  $handle = fopen($f_name,'w+');
  fwrite($handle,$data);
  fclose($handle);
   
   if($data)
   {
    
   		return true;
        }
   else
    return false;
 }  // end of the function


//  CLEAN THE QUERIES
function clean($str) {
  if(@isset($str)){
    $str = @trim($str);
    if(get_magic_quotes_gpc()) {
      $str = stripslashes($str);
    }
    return mysql_real_escape_string($str);
  }
  else{
    return 'NULL';
  }
}


?>

database backup is created after call backup_database(...) function.

Now add following code to export in .sql file format.
if($data)
  {
   header('Content-Type: application/octet-stream');   
       header("Content-Transfer-Encoding: Binary"); 
       header("Content-disposition: attachment; filename=\".$f_name."");  
       echo $data; exit;

  		return true;
       }
  else
   return false;
}  // end of the function

Now backup file is downloaded successfully

Note: Add this code in admin side only due to database security.