array variables with or without quotes

I recently encountered a situation where I had to retrieve data from a MYSQL database table with a field named "include". My code looked like this: $content_result = mysql_query('SELECT * FROM calander') or die(mysql_error()); $content_row = mysql_fetch_array($content_result); if ($content_row[include]) {go do something;} When I tried to run this, PHP treated the word -include- as a control structure rather than an index name. Understandable. So, I put it in single quotes ('include') to see what would happen and it works fine. But I don't understand why! What is the difference between $content_row [include] and $content_row['include']? I have been unable to resolve this question in the php manual, I need enlightenment. 2dogs