WordPress at a glance
function is not described

SimplePie::handle_content_type() public WP 1.0

Send the content-type header with correct encoding

This method ensures that the SimplePie-enabled page is being served with the correct mime-type and character encoding HTTP headers (character encoding determined by the {@see set_output_encoding} config option).

This won't work properly if any content or whitespace has already been sent to the browser, because it relies on PHP's header() function, and these are the circumstances under which the function works.

Because it's setting these settings for the entire page (as is the nature of HTTP headers), this should only be used once per page (again, at the top).

{} It's a method of the class: SimplePie{}

No Hooks.

Return

null. Nothing.

Usage

$SimplePie = new SimplePie();
$SimplePie->handle_content_type( $mime );
$mime(string)
MIME type to serve the page as

Code of SimplePie::handle_content_type() WP 5.7.2

<?php
public function handle_content_type($mime = 'text/html')
{
	if (!headers_sent())
	{
		$header = "Content-type: $mime;";
		if ($this->get_encoding())
		{
			$header .= ' charset=' . $this->get_encoding();
		}
		else
		{
			$header .= ' charset=UTF-8';
		}
		header($header);
	}
}