confused between ' ' and " "

Posted on 16th Feb 2014 by admin

there is a php i set:
$begin_date_query = mysql_query( "SELECT SUBDATE(due_date, INTERVAL $days_expected DAY) AS begin_date FROM baby WHERE baby_id = $_SESSION['baby_id']" );

'coz of the double quote, the variable $_SESSION['baby_id'] always be convert into string. how can i keep it as a variable, especially there is single quote go around!!! tried many combination but doesn't work. totally confused @@.

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