Function to filter HTML code under PHP_PHP tutorial

/*----------------------
过滤HTML代码的函数
-----------------------*/
function htmlEncode($string) {
    $string=trim($string);
    $string=str_replace("&","&",$string);
    $string=str_replace("'","'",$string);
    $string=str_replace("&","&",$string);
    $string=str_replace(""",""",$string);
    $string=str_replace("\"",""",$string);
    $string=str_replace("<","<",$string);
$string=str_replace("<","<",$string);
$string=str_replace(">",">",$string);
    $string=str_replace(">",">",$string);
    $string=str_replace(" "," ",$string);
    $string=nl2br($string);
    return $string;
}