Error reporting Notice:

Undefined variable: random_chars in wamp\www\php_sandbox\idgen.php on line 21

Call Stack:
#   Time    Memory  Function    Location
1   0.0045  678928  {main}( )   ..\idgen.php:0
GPB7446 

How do I fix this error?

Using this code like an automatic unique id generator. How do I connect to DB to verify code is truly unique before allowing it to be assigned to a user creating a new account?

Thanks

<?php
$characters = array(
"A","B","E","F","G","H","J","K","M","N","P","R","S","T","W","X","Y","Z");
$keys = array();
while(count($keys) < 3) {
$x = mt_rand(0, count($characters)-1);
if(!in_array($x, $keys)) {
$keys[] = $x;
}
}
foreach($keys as $key){
$random_chars .= $characters[$key];}
$randNum = rand(2327,9987); $randLet = rand(2327,9987);
echo $random_chars . $randNum;
?>