Call to undefined function imageopenpolygon()
<p>I'm trying to draw a open polygon with <code>imageopenpolygon</code>, because the normal <code>imagepolygon</code> closes automatically with the first position it has begun.Both functions exist and both syntaxes for use are identical.</p>

<p>They are officially supported:</p>

<p><a href="http://php.net/manual/pt_BR/function.imagepolygon.php" rel="nofollow noreferrer">imagepolygon</a></p>

<p><a href="http://php.net/manual/pt_BR/function.imageopenpolygon.php" rel="nofollow noreferrer">imageopenpolygon</a></p>

<p>But when I try to use <code>imageopenpolygon</code>, my PHP setup don't like it, throwing me this error:</p>

<pre><code>Fatal error: Uncaught Error: Call to undefined function imageopenpolygon() in /var/www/path/otherpath/index.php:65 Stack trace: #0 {main} thrown in /var/www/path/otherpath/index.php on line 65
</code></pre>

<p>The syntax is correct, since the other (older) one works. <code>imageopenpolygon</code> was introduced in PHP 7 and is supported until 7.2.0, stated on the site. I have:</p>

<pre><code>PHP 7.1.0-5+deb.sury.org~trusty+1 (cli) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.1.0-dev, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.1.0-5+deb.sury.org~trusty+1, Copyright (c) 1999-2016, by Zend Technologies
</code></pre>

<p>I wrote <code>phpinfo()</code> in an empty PHP file and it returned that all paths are loaded with 7.1. Image attached below:
<a href="https://i.stack.imgur.com/RPti4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RPti4.png" alt="Apache Setup"></a></p>

<p>Also, my GD is updated for PHP 7.1.</p>

<p>What can it be? I need that function and theoretically it should work, shouldn't?</p>