PHP

Runs on the server before the page reaches the browser. If you "View Source" in your browser you will never see PHP — only what it produced.

Start here

1.playing.php — Variables and a for loop
Mixing PHP into an ordinary HTML page.
 2.4.php — Echoing HTML from PHP
Why \n matters when you generate code.
 3.5.php — Building a table with a loop 4.6.php — The same table, formatted with CSS

Arrays

5.arrays.php — Arrays in PHP 6.crazyArrays.php — 2D and nested arrays 7.php-8-forEach.php — Looping an array with foreach

Files, functions, and useful odds and ends

8.php-7-fileio.php — Reading a CSV file
Reads trees.csv from this folder.
 9.uploadFiles.php — Uploading a CSV through a form 10.functionsExample1.php — Functions in a separate file
Uses require to pull in functionsExample2.php.
 11.dateStuff.php — Dates and times
date() and strtotime().
 12.json.php — Building an array and encoding it as JSON
Worth revisiting once you reach Ajax.
 13.specialFunctions.php — Inspecting the $_SERVER array