Why is $name field never !, even when empty? - php

Question

I have a basic sign up form, and when checking if a field is not set (if(!$name)) it always goes to 'else', even when the field is empty.
Does anyone know why?
(When i'm trying to check it in reverse (if($name)), it does show the error line. )
*var_dump($name) - always returns a string, never false. I'm guessing thats part of the problem?...
Thanks a lot!!
<?php
$error = '';
if(isset($_POST['submit'])){
$name = trim(filter_input(INPUT_POST, 'name', FILTER_SANITIZE_STRING));
if( !$name && preg_match("/^([\w]){2,50}$/", $name)){
$error = ' * Please enter a valid name';
}
?>
<form action="" method="post">
<label for="name">Name:</label><br>
<input type="text" name="name"> <br><br>
<input type="submit" name="submit" value="Sign up">
<span class="error"><?= $error ?></span>
</form>

Answer 1

Well, you got wrong IF statement. You are checking if $name is false (it will be only if you send empty string) AND if it's length is between 2-50.
Consider two states:
<?php
$name = 'Login';
$a = !$name; // FALSE! You're variable is OK, but you neg it
$b = preg_match("/^([\w]){2,50}$/", $name); // TRUE! Between 2-50 letters
<?php
if($a && $b) {
die("Won't be visible, because $a and $b are not the same");
}
And second:
$name = '';
$a = !$name; // TRUE! You're variable is empty, but you neg it
$b = preg_match("/^([\w]){2,50}$/", $name); // FALSE! has 0 letters
if($a && $b) {
die("Won't be visible, because $a and $b are not the same");
}
So, you should use OR instead of AND or just forget about first statement (second is checking same thing!):
<?php
if(!preg_match("/^([\w]){2,50}$/", $name)){
$error = ' * Please enter a valid name';
}

Answer 2

You're getting that undefined variable notice because you're trying to echo something in your form that hasn't already been set/not empty and your PHP/HTML form are used in the same file.
Sidenote: <?= is short tag syntax for "echo".
So change:
<?= $error ?>
to and with a conditional statement:
<?php if(!empty($error)) { echo $error; } ?>
or:
<?php if(isset($error)) { echo $error; } ?>
Or use a ternary operator:
http://php.net/manual/en/language.operators.comparison.php
or add an else to my suggestion above.
Plus, you also have a missing brace in your PHP (least, for what you posted) and that alone should have thrown you an unexpected end of file error.
Edit:
The reason that it is failing is the operator you're using, being && (AND). Use || (OR), to check if either one failed the criteria and not both. You're checking if there's no name AND if there's enough valid characters.
if( !$name || preg_match("/^([\w]){2,50}$/", $name))
and that's all it was, a simple wrong choice of operator.
Btw, my edit wasn't based on the other answer given. I actually was busy testing this out when that was posted.
Their explanation is well-written/explained, however they're working too hard.
Changing && to || in your code would have worked just fine.