<?php

$var = 'text text text text text text text text text
text text text text text text
<!-- start -->
IMPORTANT STUFF
<!-- end -->
blah blubb';

function extract_code( $str ) {
	$start	= strpos( $str, '<!-- start -->' );
	if ( $start !== false ) {
		$ret = substr( $str, $start, strlen( $str ) );
	}

	$end	= strpos( $ret, '<!-- end -->' );
	if ( $end !== false ) {
		$ret = substr( $ret, 0, $end + strlen( '<!-- end -->' ) );
	}

	return $ret;
}

print extract_code( $var );

?>