This question already has an answer here:
<?php
$greeting = 'hi';
echo 'hi' $greeting;
?>
I know that the above works when string interpolation is used such as echo "hi $greeting"; but if the code works individually as in echo 'hi' and echo $greeting I don't understand why the I get an error when the code is combined to be echo 'hi' $greeting; as I illustrated above.
</div>
Because that isn't valid code. You need to concatenate the strings if you want to do it like that:
echo 'hi' . $greeting;
You have to replace your code's this part
echo 'hi' $greeting;
to
echo 'hi'.$greeting;