#PHP 5.3, 55 bytes

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

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