Generate 6 Character Random Bit.ly like ID for a URL with PHP and MySQL

gistfile1.php

  // Set number of shortcodes to generate
  $shortcodes = 10000000;

  // Loop through and create unique shortcodes
  while ($shortcodes) {
     $shortcode = generateShortcode();
     $checkSQL  = "SELECT shortcode FROM shortcodes WHERE shortcode = '" . $shortcode . "'"; 
     $checkResult = $mysqli->query($checkSQL);

     // If doesn't exist, insert into database
     if ($checkResult->num_rows == 0) {
       $sql = "INSERT IGNORE INTO shortcodes (shortcode) VALUES ('" . $shortcode . "')";
       $mysqli->query($sql); 
       echo "Inserted Shortcode: " . $shortcode . "\r\n";
       $shortcodes--;   
     }  
  }
  
  // Function to generate shortcode
  function generateShortcode() {
    $shortcode = substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1) .
                 substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1) .
                 substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1) .
                 substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1) .
                 substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1) .
                 substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(0,61), 1); 
    return $shortcode;
  }