How can I add a condition inside a php array?

If you are making a purely associative array, and order of keys does not matter, you can always conditionally name the key using the ternary operator syntax.

$anArray = array(
    "theFirstItem" => "a first item",
    (true ? "conditionalItem" : "") => (true ? "it may appear base on the condition" : ""),
    "theLastItem" => "the last item"
);

This way, if the condition is met, the key exists with the data. If not, it's just a blank key with an empty string value. However, given the great list of other answers already, there may be a better option to fit your needs. This isn't exactly clean, but if you're working on a project that has large arrays it may be easier than breaking out of the array and then adding afterwards; especially if the array is multidimensional.

Unfortunately that's not possible at all.

If having the item but with a NULL value is ok, use this:

$anArray = array(
   "theFirstItem" => "a first item",
   "conditionalItem" => $condition ? "it may appear base on the condition" : NULL,
   "theLastItem"  => "the last item"
);

Otherwise you have to do it like that:

$anArray = array(
   "theFirstItem" => "a first item",
   "theLastItem"  => "the last item"
);

if($condition) {
   $anArray['conditionalItem'] = "it may appear base on the condition";
}

If the order matters, it'll be even uglier:

$anArray = array("theFirstItem" => "a first item");
if($condition) {
   $anArray['conditionalItem'] = "it may appear base on the condition";
}
$anArray['theLastItem'] = "the last item";

You could make this a little bit more readable though:

$anArray = array();
$anArray['theFirstItem'] = "a first item";
if($condition) {
   $anArray['conditionalItem'] = "it may appear base on the condition";
}
$anArray['theLastItem'] = "the last item";

Your can do it like this:

$anArray = array(1 => 'first');
if (true) $anArray['cond'] = 'true';
$anArray['last'] = 'last';

However, what you want is not possible.