How bad is mb_internal_encoding(“UTF-8”);?

Question

After answering Zend_Cache: After loading cached data, character encoding seems messed up

I use it to change the PHP's internal encoding , its originally ISO-8859-1,

so I need to change the encoding of every non English input value, but using it I force PHP to convert every value to UTF-8, as you might see in the question linked above.

I am Caching arabic text in files using Zend_cache, I wasn't be able to do it without that function.

I need to know: How bad is to use this function mb_internal_encoding("UTF-8");?

I had adopt to use this function in every project I opt in , all of them are using non-english characters