Interfaces may not include member variables error : <p>I made an Interface for my project which is called <code>insert</code> and goes like this:</p>

<pre><code>&lt;?php 
interface Insert
{   
    private $_db;
    public function __construct()
    {
        $this-&gt;_db = new Connection();
        $this-&gt;_db = $this-&gt;_db-&gt;dbConnect();
    }
    public function insert($table_name, $data)
    {
        $string = "INSERT INTO ".$table_name." (";
        $string .= implode(",", array_keys($data)) . ') VALUES (';
        $string .= "'" . implode("','", array_values($data)) . "')";
        if(mysqli_query($this-&gt;_db, $string))
        {
            return true;
        }
        else
        {
            echo mysqli_error($this-&gt;con);
        }
    }
}
?&gt;
</code></pre>

<p>But it given me this error:</p>

<p><strong>Fatal error: Interfaces may not include member variables</strong></p>

<p>So what is my mistake here?</p>
