Math captcha rather than text

The default captcha is cutting down spam a bit, but some is getting through.
The following has some advice...

  dapphp/securimage#66
  https://www.phpcaptcha.org/documentation/setting-options/

... but regardless of that I find the math problems as a sender less annoying
to solve. Lets give this a try for a bit.

captcha.php

@@ -0,0 +1,16 @@
+<?php
+// Our configured variant of securimage/securimage_show.php
+
+require_once dirname(__FILE__) . '/securimage/securimage.php';
+
+$img = new Securimage();
+
+$img->captcha_type = Securimage::SI_CAPTCHA_MATHEMATIC; // simple math problem
+$img->perturbation = .75;
+$img->use_random_baseline = true;
+$img->use_random_boxes = true;
+$img->num_lines = mt_rand(2, 4);
+
+if (!empty($_GET['namespace'])) $img->setNamespace($_GET['namespace']);
+$img->show();  // outputs the image and content headers to the browser
+?>

contact.php

@@ -76,11 +76,11 @@
 
             <p>Please type the text below:</p>
 
-            <img id="captcha" src="/securimage/securimage_show.php" alt="CAPTCHA image" />
+            <img id="captcha" src="/captcha.php" alt="CAPTCHA image" />
 
             <br>
             <input type="text" name="captcha_code" size="10" maxlength="6" />
-            <a href="#" onclick="document.getElementById('captcha').src = '/securimage/securimage_show.php?' + Math.random(); return false">&nbsp;<i class="fas fa-sync"></i>&nbsp;different image</a>
+            <a href="#" onclick="document.getElementById('captcha').src = '/captcha.php?' + Math.random(); return false">&nbsp;<i class="fas fa-sync"></i>&nbsp;different image</a>
 
             <p>Message:</p>
             <textarea class="contact-form" rows="11" name="message" cols="80"></textarea>