My website (php plus mysql) wants to send short messages to users' mobile phones (this function is provided by a third-party short message provider), so how can the provider provide me with this? ...

He provides you with an api interface, and you call it.

His interface will let you pass several parameters, mainly:

Mobile phone number, short message content

In addition, it is generally an encrypted interface, so in addition to the above two parameters, it will have other encryption parameters, which have been encrypted.

You need to read their development documentation.

Usually they provide examples of PHP calls, so as long as you know PHP, you don't need to worry about this.