Run something only when the file is invoked on the command line.

cli.php

<?php

# Some regular library code, which can be used by including
# the file.
class Foo
{
    # Do not use a global function, because you get an error
    # if you try to redefine it.
    static function __main()
    {    
        echo "Hey there on the command line!\n";
    }
}

# We must use realpath(), because the first argument
# only contains the relative path and __FILE__ contains
# the absolute path to this file.
if (__FILE__ === realpath($_SERVER['argv'][0])) {
    Foo::__main();
}

cli_alt.php

<?php
# Alternative version, using an anonymous function.


# Some regular library code, which can be used by including
# the file.
class Foo
{
}

# Do not use a global function, because you get an error
# if you try to redefine it.
$__main = function() {
    echo "Hey there on the command line!\n";
};

# We must use realpath(), because the first argument
# only contains the relative path and __FILE__ contains
# the absolute path to this file.
if (__FILE__ === realpath($_SERVER['argv'][0])) {
    $__main();
}

gistfile1.txt

% php cli.php
Hey there on the command line!
% php -a
% include("cli.php");
% var_export(class_exists("Foo"));
true