4 of 5 added 8 characters in body

PHP 5.3, 55 53 51 bytes

<?for(;$i<strlen($a);){echo$a{$i++}!=' '?'*':"\n";}


Usage:
Call the Script and define a global variable ($a)
php -d error_reporting=0 script.php?a="Hello World"

Output:
*****
*****