Check from functions.php if function exists in footer.php

You should not declare functions in your templates. The templates are for outputting content. The function_exists() will check if the function is declared before the function_exists() call, not after it, and templates are loaded after functions.php file, since you can control them by using the template_redirect filter.

Declare your function in your theme’s functions.php file, and then call it in your template. This is the best way to go.