将变量包含在有问题的sql查询中

I was wondering how I can include a variable to an sql query. The issue is that the code is being gathered as this way. Here is the code:

  $timestamp = mktime(0, 0, 0, date("n"), date("j") - time(), date("Y"));   
  $checkTime = date("Y.m.d", $timestamp);   

This the original sql query. Here is the code:

  $sqlResult = mysql_query("SELECT COUNT(id) FROM id WHERE timestamp = '".$checkTime."'");  

  Database timestamp entries

  2013.12.21
  2013.12.23

The code that I am using right now is written for php time epoch time, I noticed I had my variable missing. Here is the code:

  $sqlResult = mysql_query("SELECT COUNT(id) FROM id WHERE timestamp >= UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 29 DAY)) GROUP BY DAY(FROM_UNIXTIME(timestamp))");  

  Database timestamp entries

  1390537347
  1390536900

How can I incorporate my $checkTime tag into my second sql query?