#PHP 5.3, <s>55</s> 53 bytes

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

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<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>
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<br />
<b>Output:</b>
<br />
<code>*****</code>
<br />
<code>*****</code>