Description:
------------
The test script returns the following:

0 or 2 or 1 => 1
0 ?: 2 ?: 1 => 2
0 or 2 ?: 1 => 1
0 ?: 2 or 1 => 1

Is it desired?

Test script:
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<?php

$a = 0;
$b = 2;
$c = 1;
echo $a.' or '.$b.' or '.$c.' => '; echo $a or $b or $c; echo "<br>\n";
echo $a.' ?: '.$b.' ?: '.$c.' => '; echo $a ?: $b ?: $c; echo "<br>\n";
echo $a.' or '.$b.' ?: '.$c.' => '; echo $a or $b ?: $c; echo "<br>\n";
echo $a.' ?: '.$b.' or '.$c.' => '; echo $a ?: $b or $c; echo "<br>\n";

Expected result:
----------------
0 or 2 or 1 => 2
0 ?: 2 ?: 1 => 2
0 or 2 ?: 1 => 2
0 ?: 2 or 1 => 2

Actual result:
--------------
0 or 2 or 1 => 1
0 ?: 2 ?: 1 => 2
0 or 2 ?: 1 => 1
0 ?: 2 or 1 => 1