How would I create this .php file with external css?

Challenge 8: PHP Overview In this assignment, you are to create a PHP file that: Is named index.php ndex.php should have a landing page with some text o Example: "Welcome to my first php app" . Welcome to my first php app Reads parameters sent through the $_GET Super-global Array . Displays information to the browser using echo/print . Performs the following 5 functions: o https://www.professorwergeles.com/CS2830/Challenge8/ o Read about $_GET at https://www.w3schools o Read about echo/print at https://www.w3schools.com/php/php echo print.as o sayHi: superglobal URL.:../index.php?myFunction-sayHi&myName-Nick o gcd: URL: ../index.php?myFunction-gcd&x-3&y-4 o hypotenuse: URL.:../index.php?myFunction-hypotenuse&a-3&b-4 o showPic: URL: ../index.php?myFunction-showPic o showList: URL: .../index.php?myFunction showList o Note: You can click on the links above to see a live demo of each function . Each function should error check to make sure all necessary parameters are correct . If incorrect parameters are used, display a message to show an error Challenge 8: PHP Overview In this assignment, you are to create a PHP file that: Is named index.php ndex.php should have a landing page with some text o Example: "Welcome to my first php app" . Welcome to my first php app Reads parameters sent through the $_GET Super-global Array . Displays information to the browser using echo/print . Performs the following 5 functions: o https://www.professorwergeles.com/CS2830/Challenge8/ o Read about $_GET at https://www.w3schools o Read about echo/print at https://www.w3schools.com/php/php echo print.as o sayHi: superglobal URL.:../index.php?myFunction-sayHi&myName-Nick o gcd: URL: ../index.php?myFunction-gcd&x-3&y-4 o hypotenuse: URL.:../index.php?myFunction-hypotenuse&a-3&b-4 o showPic: URL: ../index.php?myFunction-showPic o showList: URL: .../index.php?myFunction showList o Note: You can click on the links above to see a live demo of each function . Each function should error check to make sure all necessary parameters are correct . If incorrect parameters are used, display a message to show an error