Transform the data into the proper format. The date you give is assumed to be in mm/dd/yyyy format, because PHP is written in the US.
If you -know- for certain that your dates will always be in dd/mm/yyyy format, try transforming it this way:
$date = date("Y-m-d H:i:s",strtotime(str_replace('/','-',$date)))
strtotime interprets x/y/z as mm/dd/yy[yy], and x-y-z as dd-mm-yyyy or yy-mm-dd, depending on whether z is 4 digits or 2.