How does PHP make text upside down?

<?PHPHeader("Content-type:text/html;charset=utf-8");$str= "1 Chinese characters utf-8 Code";/** * $b utf-8 with 3, gb2312 with 2*/functionTest$str,$b=3){           $length=strlen($str); $rets= ' ';  for($i= 0;$i<$length;$i++){         if(Ord(substr($str,$i, 1)) >128){             $rets[] =substr($str,$i,$b); $i+=$b-1; }Else{             $rets[] =substr($str,$i, 1); }       }          return implode(‘‘ ,Array_reverse($rets)); } $test= Test ($str);Var_dump($test);/**string (24) "8-FTU 1" in the word "Code series "*/

<! DOCTYPE html Public"-//w3c//dtd XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" >PHP$a= ' abcd min Love Army son ';//the test string, which contains the Chinese language///test output below, because when encoded as Utf-8, a Chinese character occupies 3 lengths//echo strlen (' min ');$b= ' ';$i=0; while($i<strlen($a)){    if(Ord(substr($a,$i, 1)) >0xa0)//The Ord function determines whether the string is Chinese    {       $b=substr($a,$i, 3).$b;//This is a very important sentence, reverse!        $i+=3; }    Else    {      $b=substr($a,$i, 1).$b; $i++; }}Echo $b;?></body>
<?php 
 Header ("Content-type:text/html;charset=utf-8"); 
 $STR = "1 Chinese characters utf-8 Code";  
 
/** 
 * $b utf-8 with 3, gb2312 with 2 
 */
 
 Function Test ($str, $b =3) {
 
 $length = strlen ($STR); 
 $rets = "; 
 
 for ($i = 0; $i < $length; $i + +) {
 if (Ord (substr ($str, $i, 1)) >128) {
 $rets [] = substr ($str , $i, $b); 
 $i + = $b-1;  
}else{
 $rets [] = substr ($str, $i, 1); 
}    
} 
 
 Return implode (", Array_reverse ($rets)); 
 
} 
 $test = Test ($STR); 
 Var_dump ($test); 
/** 
 String (24) "Code 8-FTU 1" 
 */