$this when not in object context PHPPostCode.php

Hi Guys
I want to post this code on my website. But it say:

Fatal error: Using $this when not in object context in /home/ho58c8d/public_html/Home/wp-content/plugins/php-code-for-posts/PHPPostCode.php(85) : eval()’d code on line 2

This code:

<?php
$this->db->select(“np_challenge.*, np_challenge_item.icon”);
$this->db->where(“np_challenge.id”, $challenge_id);
$this->db->join(“np_challenge_item”, “np_challenge_item.id = np_challenge.challenge_item_id”, “LEFT”);
$query = $this->db->get(“np_challenge”);
$result = $query->row();
$query->free_result();
?>

PHPPostCode.php line 85: eval( $snippetPrefix.$snippet->code );

Thanks for the help
Tuan

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