Source

File: wp-includes/formatting.php

function _wp_specialchars( $string, $quote_style = ENT_NOQUOTES, $charset = false, $double_encode = false ) {
	$string = (string) $string;

	if ( 0 === strlen( $string ) ) {
		return '';
	}

	// Don't bother if there are no specialchars - saves some processing.
	if ( ! preg_match( '/[&<>"\']/', $string ) ) {
		return $string;
	}

	// Account for the previous behaviour of the function when the $quote_style is not an accepted value.
	if ( empty( $quote_style ) ) {
		$quote_style = ENT_NOQUOTES;
	} elseif ( ENT_XML1 === $quote_style ) {
		$quote_style = ENT_QUOTES | ENT_XML1;
	} elseif ( ! in_array( $quote_style, array( ENT_NOQUOTES, ENT_COMPAT, ENT_QUOTES, 'single', 'double' ), true ) ) {
		$quote_style = ENT_QUOTES;
	}

	// Store the site charset as a static to avoid multiple calls to wp_load_alloptions().
	if ( ! $charset ) {
		static $_charset = null;
		if ( ! isset( $_charset ) ) {
			$alloptions = wp_load_alloptions();
			$_charset   = isset( $alloptions['blog_charset'] ) ? $alloptions['blog_charset'] : '';
		}
		$charset = $_charset;
	}

	if ( in_array( $charset, array( 'utf8', 'utf-8', 'UTF8' ), true ) ) {
		$charset = 'UTF-8';
	}

	$_quote_style = $quote_style;

	if ( 'double' === $quote_style ) {
		$quote_style  = ENT_COMPAT;
		$_quote_style = ENT_COMPAT;
	} elseif ( 'single' === $quote_style ) {
		$quote_style = ENT_NOQUOTES;
	}

	if ( ! $double_encode ) {
		// Guarantee every &entity; is valid, convert &garbage; into &amp;garbage;
		// This is required for PHP < 5.4.0 because ENT_HTML401 flag is unavailable.
		$string = wp_kses_normalize_entities( $string, ( $quote_style & ENT_XML1 ) ? 'xml' : 'html' );
	}

	$string = htmlspecialchars( $string, $quote_style, $charset, $double_encode );

	// Back-compat.
	if ( 'single' === $_quote_style ) {
		$string = str_replace( "'", '&#039;', $string );
	}

	return $string;
}