HTML encoding and decoding in PHP:

HTML encoding and decoding in PHP:
* https://www.php.net/manual/en/function.htmlentities.php — Convert all applicable characters to HTML entities
* https://www.php.net/manual/en/function.htmlspecialchars.php — Convert special characters to HTML entities

htmlentities(string $string, /* ... */);
htmlspecialchars(string $string, /* ... */); 
// => <p data-data='{"a":1}'>testing</p>

* https://www.php.net/manual/en/function.html-entity-decode.php — Convert HTML entities to their corresponding characters
* https://www.php.net/manual/en/function.htmlspecialchars-decode.php — Convert special HTML entities back to characters

html_entity_decode(string $string, /* ... */);
htmlspecialchars_decode(string $string, /* ... */); 
// => <p data-data='{"a":1}'>testing</p>

* It's not clear what practical difference there is between the functions which only handle special chars and the variants that don't. When in doubt, encode with htmlspecialchars (to cover just what is needed) and decode with html_entity_decode (to cover all possible cases).
* In any case additional binary flags can be supplied as the second argument to change what kind of characters are [un]escaped.