How Do I Create A Customized Random Alphanumerical Digit

Use mt_rand(min,max) to generate values.

For alphabetic values, use it in a loop three times with a range that spans 1 through 26 (or ascii value equivalents of A through Z). In either case, cunningly create character values out of them.

Then, loop through it ten times with the range (0, 9). Loop through for as many digits as you wish.

So - it should look something like:

$randString = ''; // initialize to empty

for($i=0; $i<3; $i++)
  $randString .= (your chars); // appending your three characters
for($i=0; $i<8; $i++)
  $randString .= Use mt_rand(0, 9); // appending your eight digits