Write a script that prompts the user to enter the size of the side of a square and then displays a hollow square of that size made of asterisks. Your script should work for squares of all side lengths between 1 and 20.

For making Hollow Square of Asterisks, I need to use 3 times for loop. It is easy ; check this script.
You should try it.
User will enter a value and will get exact numbers of Asterisks Hollow Square:

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 <?PHP  
      $inputs = array();  
      if (isset($_POST['submit'])) { //to check if the form was submitted  
           $value = isset($_POST['input0'])? $_POST['input0'] : null;  
           for($count = 1; $value != null; $count++){  
                array_push($inputs, $value);  
                $value = isset($_POST['input'.$count])? $_POST['input'.$count] : null;  
           }  
      }  
      var_dump($inputs);  
 ?>       
 <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">  
   <!-- to take multiple inputs copy the below line and change name="input" attribute -->  
   Enter input 1:&nbsp;<input type="text" name="input0" value="" /><br />  
   <input type="submit" name="submit" value="Submit" />  
 </form>  
 <?php  
 $less = ($inputs[0]-2);  
 for ($fl=1; $fl<=$inputs[0]; $fl++)  
   {  
     echo "*";  
     if ($fl==$inputs[0])  
       {  
         echo "<br />";  
       }  
   }  
 for ($hstr=1; $hstr<=$less; $hstr++)  
   {  
     echo "*" . str_repeat("&nbsp;", $less) . "* <br />";  
   }  
 for ($ll=1; $ll<=$inputs[0]; $ll++)  
   {  
     echo "*";  
   }  
 ?>