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.

Personally, I prefer

if ($a == $b
    && $b == $c
    && $c == $d
    && $g == $d
) {
    // code here...
}

For each line, you start with the double ampersand, indicating that the following statement is separate from the others. If you put the ampersand on the end of the line, it can become less obvious when the lines vary a lot in length.

For example;

if ($a == $b && 
    $b == $c && 
    $thisisamuchlongerstatementbecauseofthisvar == $d && 
    $g == $d
) {
    // code here...
}

In this case you have to scan the code more to know that each line is connected by a double ampersand.