Find the end date for a month

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On the first of each month I need to create a report from the previous month covering 1-15 and 16-end of month.
I am using the following code which forms part of a mssql query and part of the filenames.[PHP]$month = date('Y-m',strtotime('-1 month'));
$starts = array($month.'-01',$month.'-16');
$ends = array($month.'-15',$month.'-31');[/PHP] This creates arrays such as

Array 0=>2007-09-01,1=>2007-09-16);
Array 0=>2007-09-15,1=>2007-09-31). 

h:m:s are added inside the query. Unfortunately the query is failing in mssql if there is no 31st of the month. I don't know why.
What additional code do I need to always generate the the last day of the previous month,
ie 31 for Jan, 28 for Feb (poss leap year problem) 30 for Apr etc
Thanks

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I suppose I could use a look-up table.

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Its OK I solved it.
Used the date() 't' parameter. [PHP]$month = date('Y-m',strtotime('-1 month'));
$starts = array($month.'-01',$month.'-16');
$ends = array($month.'-15',$month.'-'.date('t',strtotime('-1 month')));[/PHP]