PHP has this mail() function to send mails. However, it's more than
plain vanilla to use it to send mails, esp if you don't have a fully
qualified domain name (FQDN), i.e. all systems on the LAN. Of course you
could use network/socket functions to connnect to the smtp port of your
mail server, but below I describe something simpler.
1. Edit /etc/mail/sendmail.cf to add your relay server:
You should have a line beginning with "DS"
Append to it your mail server.
2. Start sendmail
3. Use the mail() function, as is.
Note: Of course, this works only if the mail server allows you as a
relay host. i.e. your system must be trusted. All systems on the LAN,