update website to pdo/sqli

ive got a old site i want to update the db class. The site was sql and then was updated to work with mysqli by adding $GLOBALS["DBconnector"] which is commonly $GLOBALS["___mysqli_ston"] in a few places. This was a quick fix and ideally id like to remove them and run the site with a new db class. Some thing like replace $GLOBALS["DBconnector"] = mysqli_connect($mysql_host, $mysql_user, $mysql_pass) with $conn = new pdo/sqli($mysql_host, $mysql_user, $mysql_pass) this is with the intention to make the code cleaner with the queries pretty old fashioned like SQL_Query_exec("INSERT INTO messages (sender, receiver, added, msg, poster) VALUES(0, $invite_row[id], $dt, $msg, 0)"); to something like db-->pdo/sqli this is an example of how id like the code [login to view URL] thats for sqli id also like prepared statements added im not sure which way would be the best or easiest pdo/sqli im also unsure about about whether a new class should be added or the current [login to view URL] can be updated so i would like a idea of what anyones suggestion is before awarding the project ive removed as much as possible leaving just 13 php files and have posted on github for any one to view before taking on the job. i did think of using a orm but would prefer to keep it simple without adding vendors/atuoupload etc. And out of curiosity do i need a db class with sqli/pdo or can you use the built in functions. Thank you for reading, any other recommendations to update the code are welcome and more work may be available heres the [login to view URL]