PSR-2 standard for long if-conditions

There is no recommendation / convention for this case, and as Halcyon already mentioned this is a quite exceptional case.

However, there is a recommendation for a function call with a long list of parameters:

Argument lists MAY be split across multiple lines, where each
subsequent line is indented once. When doing so, the first item in the
list MUST be on the next line, and there MUST be only one argument per
line.

<?php
$foo->bar(
    $longArgument,
    $longerArgument,
    $muchLongerArgument
);

So if I had to create an if-statement similar to your’s, I’d do this:

if (
    $a == $b &&
    $b == $c &&
    $c == $d &&
    $g == $d
) {
    // do something
}

As you can see, this is almost the same as the solution you proposed yourself, but I prefer adding the && operators after the conditions.