One may believe an instance of class ErrorException with $severity
being 20 is more severe than another instance with $severity being 10, 
and argue that $severity of class ErrorException has nothing to do
with those error level constants, but I think this happens just
because the class is documented in a way that is not matching the
source code.

Please consider the case of using set_error_handler() without using 
class ErrorException. How could one determine the severity of an
error? The only solution is to compare the $errno with error level
constants, i.e. E_* values. These error level constants are built-in
constants provided by the PHP system. If one wants to use his own
system of determining the severity of an error, he has to define those 
constants somewhere. It is very clear that those are custom things,
and it is very strange to do so.

ErrorException is not a custom class. Anything dealing with it should 
use what the PHP system provides. In other words, it only makes sense
to use $severity of class ErrorException by comparing it with error 
level constants provided by the PHP system. And the source code says
the default value of $severity is E_ERROR.

It should be noted that larger value of $severity does not mean the 
error is more severe. The same thinking should apply to error level 
constants. However, $severity should represent the severity level of
an error, which could be determined by comparing $severity with error
level constants.

nab

Bug closed without a proper reason again?

75 = 64 + 8 + 2 + 1
Therefore, under common/practical usage, the current example
<?php
try {
    throw new ErrorException("Exception message", 0, 75);
} catch(ErrorException $e) {
    echo "This exception severity is: " . $e->getSeverity();
}
?>
is equivalent to
<?php
try {
    throw new ErrorException("Exception message", 0,
        E_CORE_ERROR | E_NOTICE | E_WARNING | E_ERROR);
} catch(ErrorException $e) {
    echo "This exception severity is: " . $e->getSeverity();
}
?>
Would anyone think it is possible or useful to do so?

Assigning to myself, to get this "severity" issue sorted out once and for all.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.