Single quotes like '' will directly print the text they contain while "" double quotes will interperet the text, such as printing the value of a variable rather than the name.

Debugging techniques that the text describes include always running through a URL, knowing the version, turning on display_errors, checking the HTML source code, reading the entire error message, and taking breaks.

$_SERVER is a predefined variable in PHP that contains server related information, such as the filename of the script being run, the browser and operating system being used, and the web app running PHP.