Issue With Isset() Function

问题

Trying to learn PHP with test and try method I am struggling with isset(). Here is the code I have:

<form action="validate.php" method="POST">
  <label title="User Input"> Custom Header Name</label>
  <input type="text" name="user_input">
  <button  type="submit" id="pageName">Submit</button>
</form>

and PHP file is as

<?php
  if(isset($_POST['user_input']))
    {
     echo "Input Is Set";
    } else{
           echo "Not Yet Set";
          }
?>

I both cases(empty or filled input) I am getting the first message "Input Is Set" as result! Can you please let me know why this is happening?

回答1:

This is because user_input have isset once form is submitted also if it is not filled, to avoid the error simply add an !empty check

if(isset(($_POST['user_input'])) && (!empty($_POST['user_input'])))

回答2:

Instead of isset, you can do this:

<?php
  if(isset($_POST['user_input']))
    {
    if (!empty($_POST['user_input'])) {
      echo "Input Is Set";
    } else {
      echo "Input is empty!";
    }
  } else{
   echo "Not Yet Set";
  }
?>

More info: http://php.net/manual/en/function.empty.php

回答3:

You can try something like

<?php
  if($_POST['user_input'] != '')
    {
     echo "Input Is Set";
    } else{
           echo "Not Yet Set";
          }
?>

Or else use if (!empty($_POST['user_input'])

Check to see if it holds any value. That should work

回答4:

You check if it's set but you should check also if it's empty.

Short version:

<?php
  echo isset($_POST['user_input']) && !empty($_POST['user_input'] ? "Input is set" : "not set";
?>

回答5:

I recommed you to work with strlen() for 2 reasons:

if (strlen($_POST['user_input']) > 0 && strlen($_POST['user_input']) < 80)
{ ... }

you can be sure the variable is not just set, but also it does actually contain data
you should cap strings from POST and GET to avoid long injected queries for example.

best is to make your own wrapper for security and isset checks in a little helper function.

来源：https://stackoverflow.com/questions/16781384/issue-with-isset-function

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