How to generate unique numeric id in php

Database systems use exclusive locking when creating numbers such as MySQL's auto_increment which takes care of concurrency and many other intricate details.

You have to approach the problem you have the same way - acquire a lock from the PHP process that's serving the request, look up the current value within some sort of persistent storage, increment it by 1, return it and release the lock.

The easiest way to do this is to use a good old file and exclusive locking.

I'll illustrate with a class [which should be debugged since it's not complete]:

class MyAutoIncrement
{
    protected $fh = null;
    protected $file_path = '';
    protected $auto_increment_offset = 1;

    public function __construct[$file_path, $offset = 1]
    {
        $this->file_path = $file_path;
        $this->auto_increment_offset = $offset;
    }

    public function autoincrement[]
    {
        if[$this->acquire[]]
        {
            $current = [int]fread[$this->fh];

            $next += $this->auto_increment_offset;

            fwrite[$this->fh, $next];

            $this->release[];

            return $next;
        }

        return null;
    }

    public function acquire[]
    {       
        $handler = $this->getFileHandler[];

        return flock[$handler, LOCK_EX];
    }

    public function release[$close = false]
    {
        $handler = $this->getFileHandler[];

        return flock[$handler, LOCK_UN];

        if[$close] 
        {
            fclose[$handler];
            $this->fh = null;
        }
    }   

    protected function acquireLock[$handler]
    {
        return flock[$handler, LOCK_EX];
    }

    protected function getFileHandler[]
    {
        if[is_null[$this->fh]]
        {
            $this->fh = fopen[$this->file_path, 'c+'];

            if[$this->fh === false]
            {
                throw new \Exception[sprintf["Unable to open the specified file: %s", $this->file_path]];
            }
        }

        return $this->fh;
    }
}

Usage:

$ai = new MyAutoIncrement['/path/to/counter/file.txt'];

try
{
    $id = $ai->autoincrement[];

    if[!is_null[$id]]
    {
        // Voila, you got your number, do stuff
    }
    else
    {
        // We went wrong somewhere
    }
}
catch[\Exception $e]
{
// Something went wrong
}