php -S localhost:8000

shell_exec('ls');
// or
exec('ls');

I'm not sure what shell you are going to get with this function, but you can find out like this:
<?php
$cmd = 'set';
echo "<pre>".shell_exec($cmd)."</pre>";
?>
On my FreeBSD 6.1 box I get this:
USER=root
LD_LIBRARY_PATH=/usr/local/lib/apache2:
HOME=/root
PS1='$ '
OPTIND=1
PS2='> '
LOGNAME=root
PPID=88057
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
SHELL=/bin/sh
IFS='    
'
Very interesting.  Note that the PATH may not be as complete as you need.  I wanted to run Ghostscript via ImageMagik's "convert" and ended up having to add my path before running the command:
<?php
$cmd = 'export PATH="/usr/local/bin/"; convert -scale 25%x25% file1.pdf[0] file2.png 2>&1';
echo "<pre>".shell_exec($cmd)."</pre>";
?>
ALSO, note that shell_exec() does not grab STDERR, so use "2>&1" to redirect it to STDOUT and catch it.

just simply type
php -a

php -f /path/to/your/file.php

php php_file.php
// or
php php_file.php > result_file.txt // to display the result of echoes or dumps

php /path/to/your/file.php

// terminal run php file
// First run "php" in the terminal, in the path directory
php
// If php works there. Then :
php path_to_file/file_name.php
//This will run your php and print stuff to your console in plain text.
//It's useful for a range of testing and data maniplation purposes.
//And has the full php cammand line. So potential is up to your imagination.

To execute a php script, use the PHP Command Line interface(CLI) and specify the file name of the script in the following way:
php script.php

cd C:\myphpscripts