How can I generate a 6 digit unique number?

$six_digit_random_number = random_int(100000, 999999);

As all numbers between 100,000 and 999,999 are six digits, of course.

If you want it to start at 000001 and go to 999999:

$num_str = sprintf("%06d", mt_rand(1, 999999));

Mind you, it's stored as a string.

Another one:

str_pad(mt_rand(0, 999999), 6, '0', STR_PAD_LEFT);

Anyway, for uniqueness, you will have to check that your number hasn't been already used.

You tell that you check for duplicates, but be cautious since when most numbers will be used, the number of "attempts" (and therefore the time taken) for getting a new number will increase, possibly resulting in very long delays & wasting CPU resources.

I would advise, if possible, to keep track of available IDs in an array, then randomly choose an ID among the available ones, by doing something like this (if ID list is kept in memory):

$arrayOfAvailableIDs = array_map(function($nb) {
    return str_pad($nb, 6, '0', STR_PAD_LEFT);
}, range(0, 999999));

$nbAvailableIDs = count($arrayOfAvailableIDs);

// pick a random ID

$newID = array_splice($arrayOfAvailableIDs, mt_rand(0, $nbAvailableIDs-1), 1);
$nbAvailableIDs--;

You can do something similar even if the ID list is stored in a database.

Here's another one:

substr(number_format(time() * rand(),0,'',''),0,6);

<?php
$file = 'count.txt';

//get the number from the file
$uniq = file_get_contents($file);

//add +1
$id = $uniq + 1 ;

// add that new value to text file again for next use
file_put_contents($file, $id);

// your unique id ready
echo $id;
?>

i hope this will work fine. i use the same technique in my website.