一行if-else

An easy question for you guys.
I assume there must be an easier (= less code) way to do the following snippet:

if (link_validate_url($items[0]['url_value'])) {
} else {
    form_set_error('', 'Not a valid URL.');
}

Simply negate the return value using the ! operator:

if (!link_validate_url($items[0]['url_value'])) {
    form_set_error('', 'Not a valid URL.');
}

Of course you could shorten it even more, but IMO that reduces the readability:

if (!link_validate_url($items[0]['url_value']))
    form_set_error('', 'Not a valid URL.');

or even

if (!link_validate_url($items[0]['url_value'])) form_set_error('', 'Not a valid URL.');

if (!link_validate_url($items[0]['url_value'])) {
   form_set_error('', 'Not a valid URL.');
}

or you can (ab)use common compiler optimization that the second part of an or is never executed if the first part is true

link_validate_url($items[0]['url_value']) or form_set_error('', 'Not a valid URL.');

Try this:

if (! link_validate_url($items[0]['url_value'])) {
    form_set_error('', 'Not a valid URL.');
}

Also, you might want to read this.

You could (ab)use the ternary operator.

Typically it is condition ? action if true : action if false; But you can leave actions blank.

This leaves you with:

link_validate_url($items[0]['url_value'] ?: form_set_error('', 'Not a valid URL.');

Edit:

This is PHP 5.3+

Notes on this operator can be found here