Why can there be encoding problems in PHP for some operations?

Question

PHP

There were problems when integrating with some services - the json_encode command returns an empty string when encoding an array to json. During debugging, it turned out that part of the data in the array is in ISO-8859-1 encoding.
The same happens when writing data to a file.
The array is formed from two sources - data from the database and data hard-core in the code in files in UTF-8.

Two different scripts behave differently - in one, the data that is added in the code and written to the file using file_put_contents in this final file is in UTF-8, and the data from the database is in ISO-8859-1.
Such an array, when trying to get json, turns into an empty string.
In the other, all data is written to the file in ISO-8859-1, and is perfectly displayed in the browser as UTF-8 and is perfectly converted to json.

I found out about the ISO-8859-1 encoding after transcoding on Lebedev's website.

Php parameters are all UTF-8. ISO-8859-1 does not appear anywhere at all.
I don't even know where to dig.

Bitrix tag despite the fact that the site is on Bitrix, but I think it doesn’t matter at all.

Answer 1

data from the database in ISO-8859-1.

database encoding not set