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Be careful with functions like strtotime() that try to "guess" what you mean (it doesn't guess of course, the rules are here).
Indeed 22-09-2008 will be parsed as 22 September 2008, as it is the only reasonable thing.
How will 08-09-2008 be parsed? Probably 09 August 2008.
What about 2008-09-50? Some versions of PHP parse this as 20 October 2008.
So, if you are sure your input is in DD-MM-YYYY format, it's better to use the solution offered by @Armin Ronacher.