How can I pass parameters from the command line to $_POST in a PHP script?

That's not easily doable. You can invoke the php-cgi binary and pipe a fake POST request in. But you'll need to set up a whole lot of CGI environment variables:

echo 'var1=123&var2=abc' | REQUEST_METHOD=POST  SCRIPT_FILENAME=script.php REDIRECT_STATUS=CGI CONTENT_TYPE=application/www-form-urlencoded php-cgi 

Note: Insufficient, doesn't work like that. But something like that...

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It's certainly easier if you just patch the script, and let it load the $_POST array from a predefined environment variable.

$_POST = parse_url($_SERVER["_POST"]);

Then you can invoke it like _POST=var=123 php script.php for simplicity.

Just insert the following lines at the beginning of your script:

/* If started from the command line, wrap parameters to $_POST and $_GET */
if (!isset($_SERVER["HTTP_HOST"])) {
  parse_str($argv[1], $_GET);
  parse_str($argv[1], $_POST);
}

This small piece of code does the trick (you may decide if you want to use $_GET or $_POST or, like I needed it, both.

After changing your script, you can call it from the command line passing your arguments:

php yourscript.php 'arg1=x&arg2=y'