In order to set project specific configuration options, simply add a php.ini file to your project, and then run the built-in server with this flag:
php -S localhost:8000 -c php.ini
This is especially helpful for settings that cannot be set at runtime (ini_set()).

It’s not mentioned directly, and may not be obvious, but you can also use this to create a virtual host. This, of course, requires the help of your hosts file.
Here are the steps:
1 /etc/hosts
 127.0.0.1 www.example.com
2 cd [root folder]
 php -S www.example.com:8000
3 Browser:
 http://www.example.com:8000/index.php
Combined with a simple SQLite database, you have a very handy testing environment.

I painfully experienced behaviour that I can't seem to find documented here so I wanted to save everyone from repeating my mistake by giving the following heads up:
When starting php -S on a mac (in my case macOS Sierra) to host a local server, I had trouble with connecting from legacy Java. 
As it turned out, if you started the php server with 
"php -S localhost:80" 
the server will be started with ipv6 support only!
To access it via ipv4, you need to change the start up command like so:
 "php -S 127.0.0.1:80"
which starts server in ipv4 mode only.

If your URI contains a dot, you'll lose the $_SERVER['PATH_INFO'] variable, when using the built-in webserver.
I wanted to write an API, and use .json ending in the URI-s, but then the framework's routing mechanism broke, and it took a lot of time to discover that the reason behind it was its router relying on $_SERVER['PATH_INFO'].
References:
https://bugs.php.net/bug.php?id=61286 

To output debugging information on the command line you can write output to php://stdout:
<?php
$path = $_SERVER["SCRIPT_FILENAME"];
file_put_contents("php://stdout", "\nRequested: $path");
echo "<p>Hello World</p>";
?>

On Windows you may find useful to have a phpserver.bat file in shell:sendto with the folowing:
explorer http://localhost:8888
rem check if arg is file or dir
if exist "%~1\" (
 php -S localhost:8888 -t "%~1"
) else (
 php -S localhost:8888 -t "%~dp1"
)
then for fast web testing you only have to SendTo a file or folder to this bat and it will open your explorer and run the server.

Listen on all addresses of IPv4:
php -S 0.0.0.0:80
Listen on all addresses of IPv6:
php -S [::0]:80

To send environment variable as long as with PHP built-in web server, type like this.
~$ MYENV=dev php -d variables_order=EGPCS -S 0.0.0.0:8000
On PHP script we can check with this code.
<?php
 echo getenv('MYENV'); // print dev