Question

The following code:

<?php//comment
error_reporting(0);

results (at least in 5.2.11) in:

Parse error: syntax error, unexpected T_STRING

… which I find quite bizarre.

Whether or not <?php//comment should be allowed verbatim, I propose that there can be no doubt as to the bugginess of the contents of the parse error.

That’s because there is no T_STRING in sight (though I do understand that productions may form other productions to imbue precedence, e.g. additive-expressions encapsulating multiplication operations, and the like).

Can anyone with an understanding of the PHP grammar explain how this has come about?

Answer 1

See the following snippet which looks for the <?php opening tag. Note the requirement of a trailing space, horizontal tab or new line.

<INITIAL>"<?php"([ \t]|{NEWLINE}) {
    <snip>
    return T_OPEN_TAG;
}

^{(Source: zend_language_scanner.l from PHP 5.3.10)}

This means that PHP doesn’t see your code as a normal (long) open tag, but rather as having a short open tag.

<INITIAL>"<?" {
    if (CG(short_tags)) {
        <snip>
        return T_OPEN_TAG;
    } else {
        goto inline_char_handler;
    }
}

^{(Source: zend_language_scanner.l from PHP 5.3.10}

The short tag does not require the trailing whitespace.

So, the different parts of your code that PHP sees are <?, php, //comment, error_reporting…. The php is taken to be a constant (which doesn’t exist, see below). Then there is a comment, which is fine. However the next step is to call the error_reporting() function. This is not something that is allowed to happen at this point and it is now that PHP bails out, complaining that the identifier (the T_STRING) error_reporting was not expected.

To demonstrate this, lets try some code that does work (meaning: PHP code is executed successfully) with short_open_tag enabled, but won’t with short_open_tag disabled.

<?php//comment
;

With error_reporting high enough to show notices, you’ll be able to see what goes wrong as the output is something like:

Notice: Use of undefined constant php – assumed ‘php’ in example.php on line 2

With short_open_tag turned off, the output will be the code in plain text since PHP does not try to execute it.

Finally, a little reminder of the description about T_STRING in the List of Parser Tokens which says that T_STRING is for (don’t ask me why it’s not T_IDENTIFIER, T_BAREWORD or something less confusing):

identifiers, e.g. strings, object oriented keywords like parent and self, functions, classes and more are matched.