I read in the documentation, that the set_error_handler function helps us to perform some actions when a critical error happens. I wrote an error handler function named myFunction and set set_error_handler("myFunction"), however it works only when a Notice or a Warning occurs. When a fatal error happens (to call an undefined function), the PHP error handler is executed:
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<?php
function myFunction($a,$b,$c,$d){
switch($a){
 case 1: $a="Fatal error:";break;
 case 2: $a="Warning:";break;
 case 8: $a="Notice:";
}
echo "<b>$a</b> $b iniside <b>$c</b> on line<b> $d</b>";
}

set_error_handler("myFunction");
echo undef();
?>
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The settings in the php.ini file:

; log errors = On  
; error_log = D:/inetpub/wwwroot/visit.txt
; error_log = syslog
; all three are disabled ...