As explained on http://php.net/default-charset,

the “UTF-8” is the default value and its value is used as the default
character encoding for htmlentities(), html_entity_decode() and
htmlspecialchars() if the encoding parameter is omitted.

It is set on default php.ini as “UTF-8” on the “Data handling” section as:

; PHP's default character set is set to UTF-8.
; http://php.net/default-charset
default_charset = "UTF-8"

Also, you can set, before the content, the header to another encoding as needed:

header('Content-Type: text/html; charset=utf-8');

or

header('Content-Type: text/html; charset=iso-8859-1');

or any other charset you need to declare.