What actually happens after php interpreter parses code?

What actually happens in the background after php interpreter parses code that says, open a database connection, execute curl request, etc.? So when a php script is run and the php interpreter parses the script and finds code that says open a database connection and perform a query, how does this actually happen? What goes on in the background that actually fulfills this request and performs the communication with the database?

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You can browse the source code for PHP here, that should give you some insight into its internal workings.