Increments.php

<?php

namespace App;

use Illuminate\Database\QueryException;

class Increments
{
    public static function generate($key)
    {

        // we need a transaction because lockForUpdate only
        // locks the row when used inside a transaction

        return \DB::transaction(function () use ($key) {

            // if no rows exist there is no row to lock
            // so we ALWAYS try to insert an empty row
            // as the first thing, as it's the only way
            // to ensure absolute consistency between
            // concurrent requests.
            //
            // since "key" is the primary key of the table
            // it will always fail except the first time

            try {
                \DB::table('increments')->insert([
                    'key' => $key,
                    'value' => 0,
                ]);
            } catch (QueryException $e) {

                // we need to use a try catch because Laravel
                // does not support INSERT IGNORE and alikes
            }

            // we can now lock the row

            $record = \DB::table('increments')
                ->where('key', $key)
                ->lockForUpdate()
                ->first();

            // increment the counter

            $value = $record->value + 1;

            // write it

            \DB::table('increments')
                ->where('key', $key)
                ->update(['value' => $value]);

            // and hand it to the user

            return $value;
        });

    }
}