What are the specific uses of the common functions included in the php-system? Can any expert explain them to me?

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);

<code>         return $content;        }</code>

?> ;

Reply content:

String replacement, filter html code