create superglobal variables in php?

Here's what worked best for me when trying to script this (in case anyone else comes across this like I did):

$ pecl -d php_suffix=5.6 install <package>
$ pecl uninstall -r <package>

$ pecl -d php_suffix=7.0 install <package>
$ pecl uninstall -r <package>

$ pecl -d php_suffix=7.1 install <package>
$ pecl uninstall -r <package>

The -d php_suffix=<version> piece allows you to set config values at run time vs pre-setting them with pecl config-set. The uninstall -r bit does not actually uninstall it (from the docs):

vagrant@homestead:~$ pecl help uninstall
pecl uninstall [options] [channel/]<package> ...
Uninstalls one or more PEAR packages.  More than one package may be
specified at once.  Prefix with channel name to uninstall from a
channel not in your default channel (pecl.php.net)

Options:
  ...
  -r, --register-only
        do not remove files, only register the packages as not installed
  ...

The uninstall line is necessary otherwise installing it will remove any previously installed version, even if it was for a different PHP version (ex: Installing an extension for PHP 7.0 would remove the 5.6 version if the package was still registered as installed).

Writing the variables to a specific memory address and reading and writing to/from that address with information such as a count might work.

An (old) article on IPC (inter process communication) might help you

http://zez.org/article/articleview/46/

Out of interest why is it you don't want to use files or a database?

Another solution (despite more than likely technically using files or a database) would be to force the script to use a specific session ID.

<?php

$maximum = 1;

session_id(md5('myscript'));
session_start();

if( !isset( $_SESSION['count'] ) ) {
    $_SESSION['count'] = 1;
} else {
    if( $_SESSION['count'] >= $maximum ) {
        die( "too many processes running" );
    } else {
        $_SESSION['count']++;
    }
}

session_write_close();

// simulate running something
sleep(10);

session_id(md5('myscript'));
session_start();

$_SESSION['count']--;

session_write_close();

echo "<br />executed";

?>

Note in the example above, the call to session_write_close() is important because the execution of the script will prevent the counter being incremented until it finishes otherwise. And then as it closes it, it needs reopening to decrement the counter.

Obviously because of the session reopening, any output from the script would need to be buffered (see http://www.php.net/ob_start) to prevent headers already sent errors