Many developers use md5 or rand() function to generate the unique id in php, Which is actually not a good practice under some senario. The best technique is to use the pre given php function whic is uniqid()
Example:
// generate unique string
echo uniqid();
/* prints
4bd67c947233e
*/
// generate another unique string
echo uniqid();
/* prints
4bd67c9472340
*/
// with prefix
echo uniqid('ilovephp_');
/* prints
ilovephp_4bd67d6cd8b8f
*/
// with more entropy
echo uniqid('',true);
/* prints
4bd67d6cd8b926.12135106
*/
// both
echo uniqid('phpfresher_',true);
/* prints
phpfresher_4bd67da367b650.43684647
*/
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