Passing a path in a PHP CLI script argument

Passing a path in a PHP CLI script argument

I'm writing a php CLI script that accepts, among other argument, a path.
So an example is:
php myscript.php -p=/Volumes/Macintosh HD/Users/andrea/samples
The script has his own way to red the arguments and it properly gets the
value for -p, setting it in a variable called $project_path. However, when
I test the folder with isdir($project_path) it returns false.
I've tried to pass the path in different ways:
/Volumes/Macintosh\ HD/Users/andrea/samples
'/Volumes/Macintosh HD/Users/andrea/samples'
"/Volumes/Macintosh HD/Users/andrea/samples"
'/Volumes/Macintosh\ HD/Users/andrea/samples'
"/Volumes/Macintosh\ HD/Users/andrea/samples"
Non of them works. What's the format I must use to make it work? Please
consider that the script must also work on different OS (i.e. Windows).