Php $_Post doesn't work after certain length imput - php

Question

I have a form
<form action="welcome.php" method="post" ........................">
<button id=......................ERATE NEW ....</button>
<br/><br/>
<div class="form-field">
<label for...............">....... TEST</label>
<br/>
<textarea id=...........................="false"></textarea>
<span class="clipSp............ successfully copied to clipboard" ></span>
<br/>
</div>
<br/>
<div clas..........content">
<div>ADDRESS
<br/>
<span>{{vm.displayAddress}}</span>
</div>
</div>
<button class="wButton fade" type="submit">REGISTER ACCOUNT</button>
<span class="divider-2"></span>
<button class="wButton fade" type="reset" ng-click="vm.back()">BACK</button>
.
.
.
.
Then i have this "welcome.php" file.
if(!empty($_POST['formdesiredpost'])){
$var = $_POST['formdesiredpost'];
file_put_contents("dat‌​a.txt", $var . "\n", FILE_APPEND);
header("Location: https://randomwebsiteredirect.com");
exit();
}
Its supposed to post the desired value from the form (textarea) into a txt file, data.txt and then redirect to a website.
The problem is that it only works for short length inputs. I mean aprox (15 characters) , if I input more than 15 , like 70, it simply redirects to blank welcome.php and does nothing.
Any clue of what may be happening?

Answer 1

On your html you don't have the "formdesiredpost" field. Please check does there exist "formdesiredpost" field.
And in your welcome.php, on the top side of your code add error reporting function:
error_reporting(0);
To see what happened after send data to welcome.php
Works fine:
index.php:
<form action="welcome.php" method="post">
<textarea name="formdesiredpost"></textarea>
<input type="submit" value="submit">
</form>
welcome.php
<?php
if(!empty($_POST['formdesiredpost'])){
$var = $_POST['formdesiredpost'];
file_put_contents("data.txt", $var . "\n", FILE_APPEND);
header("Location: https://randomwebsiteredirect.com");
exit();
}
?>
And please check your character limits of fields:
Limit number of characters allowed in form input text field