系统包含的常用函数具体有什么用哪位大神能帮帮忙解释一下吗？

<?php
function unhtml($content){
$content=str_replace("&","&",$content);
$content=str_replace("<","<",$content);
$content=str_replace(">",">",$content);
$content=str_replace(" "," ",$content);
$content=str_replace(chr(13),"
",$content);
$content=str_replace("\","\\",$content);
$content=str_replace(chr(34),""",$content);

         return $content;
        }

?>

字符串替换，过滤html代码