When a user signs up at your website, you may want to verify their email address so you aren't getting bogus accounts in your database. A few lines of code can send an email to the address specified, containing a welcome greeting and link to activate their account. PHP is the best way to both send the emails and do the activation process.
When the link in the email is clicked, a PHP page is loaded that performs a couple simple validation techniques, and if valid, it activates the account. PHP checks for the email address in the database and makes sure the random variable attached to the URL is the one generated at account creation to be used for activation. If these conditions are met, the account is activated.
Some sites also allow the user to resend the activation email, which makes the account activation code a little longer. PHP then has to delete the value for the old activation variable and insert a new randomly generated variable in the database. The script also has to send the email again, but with a different messaged attached - like "This is a resent confirmation email to activate your account with us".
I am a PHP website developer at http://www.simplyphp.com with 6 years experience.