Source

File: wp-includes/formatting.php

function wp_encode_emoji( $content ) {
	$emoji = _wp_emoji_list( 'partials' );

	foreach ( $emoji as $emojum ) {
		$emoji_char = html_entity_decode( $emojum );
		if ( false !== strpos( $content, $emoji_char ) ) {
			$content = preg_replace( "/$emoji_char/", $emojum, $content );
		}
	}

	return $content;
}