I am a beginner with PHP, but I wish to send mails to @yahoo.com, @gmail.com and the other emails such as these. I have read some lessons, however i don`t possess a SMTP server (I do not know what's that), but I have read somewhere that you'll be able to send via GMail (smtp.gmail.com). How's this possible ?

I am running an Apache server on Home windows 7.

I've implemented msmtp.sourceforge.net inside my Home windows 7 atmosphere coupled with a google account, works wonderful.

The configuration file you'll need for Gmail is as follows:

A system wide configuration is optional.
     # If it exists, it usually defines a default account.
     # This allows msmtp to be used like /usr/sbin/sendmail.
     account default

     # The SMTP smarthost.
    host smtp.gmail.com
    domain smtp.gmail.com
    tls on
    tls_certcheck off
    tls_starttls on
    auth on
    user user@domain.co.uk
    from user@domain.co.uk
    password yourpasswordhere
    port 587
    logfile C:\msmtp\msmtplog.txt

     # Construct envelope-from addresses of the form "user@oursite.example".
     auto_from on
     maildomain user@domain.co.uk

Additional information can be obtained inside the documentation deliver to msmtp but essentially using the downloaded file, this configuration along with a slight adjustment towards the php.ini file you ought to be all set.

You should use PHPMailer to get it done. This is a tutorial