Description:
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The documentation for set_error_handler() seems to be wrong:

<?php
function handler($errno, $errstr, $errfile, $errline, $errcontext) {}

var_dump(set_error_handler('handler'));
var_dump(set_error_handler('handler'));

NULL
string(7) "handler"

So NULL is not used for errors. NULL means the default error handler
here apparently. Whereas the documentation reads:

"Returns a string containing the previously defined error handler (if
any), or NULL on error. If the previous handler was a class method,
this function will return an indexed array with the class and the
method name."

The code reads:

"Returns the previously defined error handler, or false on error."