Hey Angelina,
Is the image actually there? maybe add an alt tag to your <img> tag so that will show up if the path to the image is broken.
Next thing is does the database have some numbers in the review_rating field - looks to me like the function only inserts the image tag if the review_rating field is >0.
You could as a debugging thing have your function return "!" . $movie_rating. If you can see the exclamation mark then its definitely getting called.
Let us know if thats any help.
cheers,
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