Source

File: wp-includes/rss.php

function __construct( $source ) {

		# Check if PHP xml isn't compiled
		#
		if ( ! function_exists('xml_parser_create') ) {
			return trigger_error( "PHP's XML extension is not available. Please contact your hosting provider to enable PHP's XML extension." );
		}

		$parser = xml_parser_create();

		$this->parser = $parser;

		# pass in parser, and a reference to this object
		# set up handlers
		#
		xml_set_object( $this->parser, $this );
		xml_set_element_handler($this->parser,
				'feed_start_element', 'feed_end_element' );

		xml_set_character_data_handler( $this->parser, 'feed_cdata' );

		$status = xml_parse( $this->parser, $source );

		if (! $status ) {
			$errorcode = xml_get_error_code( $this->parser );
			if ( $errorcode != XML_ERROR_NONE ) {
				$xml_error = xml_error_string( $errorcode );
				$error_line = xml_get_current_line_number($this->parser);
				$error_col = xml_get_current_column_number($this->parser);
				$errormsg = "$xml_error at line $error_line, column $error_col";

				$this->error( $errormsg );
			}
		}

		xml_parser_free( $this->parser );
		unset( $this->parser );

		$this->normalize();
	}