This basic snippet will create a random authentication code, or just a random string.
?php
# This particular code will generate a random string
# that is 25 charicters long 25 comes from the number
# that is in the for loop
$string = "abcdefghijklmnopqrstuvwxyz0123456789";
for($i=0;$i<25;$i++){
$pos = rand(0,36);
$str .= $string{$pos};
}
echo $str;
# If you have a database you can save the string in
# there, and send the user an email with the code in
# it they then can click a link or copy the code
# and you can then verify that that is the correct email
# or verify what ever you want to verify
?>