What is the reason I can't execute the code below? : <pre><code>&lt;?php

$greeting = 'hi';

echo 'hi' $greeting;
?&gt;
</code></pre>

<p>I know that the above works when string interpolation is used such as <code>echo "hi $greeting";</code> but if the code works individually as in <code>echo 'hi'</code> and <code>echo $greeting</code> I don't understand why the I get an error when the code is combined to be <code>echo 'hi' $greeting;</code> as I illustrated above.</p>
