Allows you to display a form field based on a condition which allows you to do something like "only display the field when value of field 'foo' is 'bar' and 'bla' is 'yes'".

$foo == 'bar' && $bla == 'yes'

You can also check the array (e.g. multiple checkboxes or select menu):

in_array('bar', $foo)

To validate a single checkbox simply compare its value:

$foo == '1'