Undefined offset 1

I am facing a problem that undefined offset :1 in line 3. I can't understand that what type of error it is. Can anyone tell me that why such error occurs in php

Undefined offset in line : 3

    foreach ($lines as $line)
    {
      list($var,$value) = explode('=', $line); //line 3
      $data[$var] = $value;
    }