如何验证echo内的文本框？

I'm trying that validating the text box inside echo in php. I tried some methods but it is not working.

php form code:

 echo '<form method="post" action="singlepage.php?id='.$idn.'"';
 echo '<label id="blogtextarea2">Comment:</label><textarea rows="10" cols="75" name="comment" id="blogtextarea" ></textarea><br>';
 echo '<input type="submit" name="post" id="blogsubmit" value="post">';
 echo '</form>';

and validation code is

if(isset($_POST['post']))
 {
if ($_POST['comment'] != "") {
        $_POST['comment'] = filter_var($_POST['comment'], FILTER_SANITIZE_STRING);
        if ($_POST['comment'] == "") {
            $errors .= 'Please enter a valid name.<br/><br/>';
        }
    } else {
        $errors= 'Please enter your name.<br/>';
    }
  }                       

Ok found it why it is not working

if(isset($_POST['post']))
{
    if (!empty($_POST['comment'])) {
        $_POST['comment'] = filter_var($_POST['comment'], FILTER_SANITIZE_STRING);
    } else {
        $errors = 'Please enter your name.';
    }
}   
echo $errors;

You have to check defined conditions on singlepage.php

I'm not exactly sure what you meant by "not working", but I have cleaned up your code and changed the check for an empty POST comment data using the empty() function instead.

There was also a redundant check within your first check for an empty comment which I've removed:

$errors = '';
if(isset($_POST['post'])) {
    if(!empty($_POST['comment'])) {
        $_POST['comment'] = filter_var($_POST['comment'], FILTER_SANITIZE_STRING);
    } else {
        $errors = 'Please enter your name.<br/>';
    }
}
echo $errors;