PHP Generate Unique Order Number with Date

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I want to generate unique identificator in following 12 numeric format: YYYYMMDDXXXX Example: 201403052318 Where: YYYYMMDD is a current date value and other XXXX is randomly generated value. $today = date("Ymd"); $rand = sprintf("%04d", rand(0,9999)); $unique = $today . $rand; Daily required unique volume is about 100. What methods using PHP should I use to prevent possible duplicates in rand values or make all id maximum unique? Maybe possible use current time functions to compact these numbers in last 4 characters? EDITED: Unique value connected to MySQL database as prime index of table. It is initial values not connected to any stored information in database. 資料來源