Why does missing "require" / "include" call error_handler an extra time? - include

Question

I have a custom error handler set up using set_error_handler. When I tried to include a file that doesn't exist, PHP calls error_handler one more time than it should:
<?php
error_reporting(E_ALL | E_STRICT);
set_error_handler(function($errno, $errstr, $errfile, $errline, $errcontext){
if(error_reporting() !== 0){
echo "<br>";
echo "<br>In Custom Error Handler...";
echo "<br>Err String: ", $errstr;
echo "<br>Passing to Default Handler...";
}
return false; // allow default
});
include("/missing_file.php"); // line 11
?>
Output:
In Custom Error Handler... // this is the extra error handler call
Err String: include(/missing_file.php)
[function.include]: failed to open stream: No such file or directory
Passing to Default Handler...
// the default handler does nothing, even though error_reporting is not zero
// Next Phase:
In Custom Error Handler...
Err String: include() [function.include]: Failed
opening '/missing_file.php' for inclusion
(include_path='.:/usr/lib/php:/usr/local/lib/php')
Passing to Default
Handler...
Warning: include() [function.include]: Failed opening
'/missing_file.php' for inclusion
in
/home/yccom/public_html/apr/test.php on line 11
The same behavior is observed with require.
For example, changing line 11 to require will give this output:
In Custom Error Handler... // this is the extra error handler call
Err String: require(/missing_file.php) [function.require]: failed to
open stream: No such file or directory
Passing to Default Handler...
// the default handler does nothing, even though error_reporting is
not zero
// Next Phase:
Fatal error: require() [function.require]: Failed opening required '/missing_file.php' in /home/yccom/public_html/apr/test.php on
line 11
What may be causing the error handler's additional call?

Answer 1

It's quite simple, really. PHP's lifecycle consists of 4 distinct phases:
Parsing
Compilation
Scanning
Execution
for your code to be parsed, all files that are included/required need to be fetched in the first phase, to translate the code into meaningful expressions. Your file doesn't exist, so a warning is issued.
Next, the compilation phase encounters the same include statement, and tries to convert the expressions into opcodes. The file doesn't exist, so a warning is issued.
Scanning translates code into tokens, which again cannot be done for the missing include file.
Execution time... The file cannot be executed because it is missing.
Why would PHP work like this? Isn't it stupid to blunder along, eventhough a file is missing?
In a way, yes, but include is used to include files that are non-critical to the script, if you really need that file's contents, you use require (but preferably require_once). The latter emits, as you stated, a fatal error, and stops everything dead in its tracks. That's what should happen if you're depending on another file for your code to function.
The require construct issues an E_COMPILER_ERROR, which effectively halts the compiler (not unlike __halt_compiler) at a given offset(the line where the failing require statement resides).
Check these slides for more details on each of the 4 main phases.
The reason why your code emits four warnings, is simply because PHP tries to include the file four times. Try running the script from the command line, but use strace:
$ strace -o output.txt php yourScript.php
open the output file, and see the internals of the Zend engine. Pay special attention to lines that look like:
lstat("/your/path/./file.php", 0x50113add8355) = -1//0x5... ~= 0xsomeaddress
You'll see where PHP goes looking for the file: it's all its include_path directories, the cwd, /usr/share/php, probably a pear or lib directory, and the include path you explicitly set.
I've gotten the idea to do this from this site, and based on the output I got, this seems to me to be the most plausible explanation as to why you see multiple errors.