Randomly add a specific string of php code anywhere in the article

Randomly add a specific string of php code anywhere in the article, usage tutorial: http://www.tongqiong.com/read.php?tid-102.html

/*Assume your text variable is named $str, and the code variables that need to be inserted are named $ad[1], $ad[2], .....
Author: http ://www.tongqiong.com
*/
$ad[1]="Tongqiong Forum";
$ad[2]="Where to shop";
$ad[3]="Taizhou Mom Forum";
 $ad[4]="Glasses Forum";
$textlength = strlen($str);
$curpos = ($textlength >= 100) ? rand(30, 100) : ($textlength > 30 ? rand( 30, $textlength) :$textlength);
$curadid = 1;
while($curpos < $textlength){
 $str = substr($str, 0, $curpos).$ad[$curadid].substr ($str, $curpos, $textlength);
 $curpos += rand(20, 50) + strlen($ad[$curadid]);
 if($ad[$curadid + 1]) $curadid += 1 ;
}

/*There is something you didn’t mention here. What if the text is not 30 characters long? It's impossible to insert between 30 and 100. So I added a judgment and added it to the end if it was less than 30.
Author: http://www.tongqiong.com

In addition, the article may be longer, such as 10,000 characters, but there are only 2 pieces of code. My writing method will automatically insert code 2, if there is no code 3.
*/
?>