How to use $_SERVER['REQUEST_URI']

Without quotes PHP interprets the REQUEST_URI as a constant but corrects your typo error if there is no such constant and interprets it as string.

When error_reporting includes E_NOTICE, you would probably get an error such as:

Notice: Use of undefined constant REQUEST_URI - assumed 'REQUEST_URI' in <file path> on line <line number>

But if there is a constant with this name, PHP will use the constant’s value instead. (See also Array do's and don'ts)

So always use quotes when you mean a string. Otherwise it can have unwanted side effects.

And for the difference of single and double quoted strings, see the PHP manual about strings.

The first one is wrong - you're actually looking for a constant REQUEST_URI that doesn't exist. This will generate a notice-level warning.

There's no difference between the other two.

There is a difference between single and double quotes in PHP string handling. A string enclosed in double quotes will be evaluated for embedded variables and escape characters (e.g. \n); a string enclosed in single quotes won't (or not as much).

So, for example,

$hello = "world";

echo "Hello $hello!\n";
echo 'Hello $hello!\n';
echo 'Done';

will output

Hello world!
Hello $hello!\nDone

In situations where you have no escape characters or embedded variables, it is slightly more efficient to use single quotes as it requires less processing of the string by the runtime. However, many people (me included) prefer to use double quotes for all strings to save confusion.