Question

I want to write a php a function which will create 50000 unique random alpha numeric string of length 4 and insert it into a db table. how can I do it?

Answer 1

for($i = 0; $i < 50000; $i++)
    $pdo->exec("INSERT INTO table_x (the_string) VALUES (UUID());");

As documented here

Edit: alternative insert manner (striping the dashes)

for($i = 0; $i < 50000; $i++)
    $pdo->exec("INSERT INTO table_x (the_string) VALUES (REPLACE(UUID(), '-', ''));");

Edit worth mentioning:

Within one single server the UUIDs encode geographic location and precision time, along with sha-1 random values.

Thus the probability of collision only exists within separate servers (for example when merging their data sets).

So long as we don’t overflow the capacity of the geo/time slots it is guaranteed to not create duplicate values locally.

As a matter of optimisation (speed of database reads) casting the UUID to a binary(16) field (and having the column in the table to match that datatype) is faster and more compact.