PHP strtotime() "first monday february" returns second monday if february 1st is a monday

Looks like you are just missing an "of":

echo date('Y-m-d', strtotime('first monday of february 2010'));

will give the expected result. See the PHP Manual on Relative dates for the various input formats.

Depending on your version of PHP the 'of' statement may or may not work. As another solution try:

echo date('Y-m-d',strtotime('monday February 2010'));

will return the first monday of February 2010. Works for all days as well.