Generate unique coupon codes

coupons.php

<?php
  //prefi for coupons
	$prefix = 'ABCD';
	
//how many codes you need?
	$need = 200;
	$codes = array();
	
	while(count($codes)!=$need){
    //choose format 
		$code = $prefix.getRandomStringNUMS(2).getRandomStringCHARS(2).getRandomStringNUMS(2).getRandomStringCHARS(2);
		$codes[$code] = $code;
	}
	
	function getRandomStringCHARS($length = 3){
		$validCharacters = "ABCDEFGHIJKLMNOPQRSTUXYVWZ";
		$validCharNumber = strlen($validCharacters);
		$result = '';
		for($i =0; $i < $length; $i++)
		{
			$index = mt_rand(0,$validCharNumber-1);
			$result .= $validCharacters[$index];
		}
		return $result;
	}
	
	function getRandomStringNUMS($length = 3){
		$validCharacters = "1234567890";
		$validCharNumber = strlen($validCharacters);
		$result = '';
		for($i =0; $i < $length; $i++)
		{
			$index = mt_rand(0,$validCharNumber-1);
			$result .= $validCharacters[$index];
		}
		return $result;
	}
	
	foreach($codes as $c){
		echo $c."\n";
	}