How to get "key" from "value" in custom meta

i have a single post’s custom meta, ex:
custom meta key = moviename
custom meta value = HarryPotter

i can use code:
<?php echo get_post_meta($post->ID, ‘moviename’, true); ?>
to get the post’s custom meta value “HarryPotter”

but, how can i get the custom meta key “moviename”,
with the value “HarryPotter” that i know?

please help me!