Fatal error: Call to a member function url() on a non-object on line 8

I am new to PHP, currently getting error saying

Fatal error: Call to a member function url() on a non-object on line 8

Below is the code I am trying

<?php
   $subpages = $site->pages()->children()->visible();
   $image_url = $subpages->image()->url();
   $title = $subpage->title()->text();

   foreach($subpages as $subpage) {
      echo '<div class="col-md-4">';
      echo '<h2>' . $title . '</h2>';
      echo '<a href="' . $subpage->url() . '" title="' . $title . '">';
      echo '<img src="' . $image_url . '" alt="' . $title . '" class="img-responsive img-thumbnail">';
      echo '</a>';
      echo '</div>';
   }
?>

Solution

Here's your code corrected:

<?php
   $subpages = $site->children()->visible();
   foreach($subpages as $subpage) {
   $image_url = $subpage->image()->url();
   $title = $subpage->title()->html();
      echo '<div class="col-md-4">';
      echo '<h2>' . $title . '</h2>';
      echo '<a href="' . $subpage->url() . '" title="' . $title . '">';
      echo '<img src="' . $image_url . '" alt="' . $title . '" class="img-responsive img-thumbnail">';
      echo '</a>';
      echo '</div>';
   }
?>

What's wrong with your code:

You should use $site->children() to list all children of the site See kirby docs
You're defining $image_url and $title before the foreach which is not correct. I moved them just at the beginning of the foreach loop. Also corrected image_url to use subpage instead of subpages.
You're using text() on your title. That doesn't exist, use either kirbytext() or html() depending on what you want to do. See the docs.