#PHP 5.3, <s>55</s> <s>53</s> 51 bytes

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

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<b>Usage:</b>
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Call the Script and define a global variable ($a)
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<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>