What sort of issue would generate an E_COMPILE_WARNING in PHP?

Question

I've spent a bit searching The Google as well as SO and have been unable to find an answer for this simple question:

What sort of issue would generate an E_COMPILE_WARNING in PHP?

The PHP manual entry on the subject ^docs says of E_COMPILE_WARNING:

Compile-time warnings (non-fatal errors). This is like an E_WARNING, except it is generated by the Zend Scripting Engine.

But I'm not sure what this would constitute. What's the difference between a regular E_WARNING and a warning raised by the Zend Scripting Engine? Could someone please explain and provide a code snippet if applicable?

Answer 1

Basically, the difference between an E_WARNING and an E_COMPILE_WARNING is that E_COMPILE_WARNING is generated while the script is still compiling.

E_COMPILE_WARNING is similar to E_COMPILE_ERROR in that it is generated during compile time, but an E_COMPILE_WARNING does not prevent script execution the way E_COMPILE_ERROR does. Compare it to the the relation between E_ERROR and E_WARNING, where the former halts execution, and the latter allows execution to continue.

For example, the following code generates an E_COMPILE_WARNING:

<?php
    echo "\n";
    echo "Hello World";
    echo "\n\n";
    var_dump(error_get_last());
    declare(foo='bar');
?>

output:

Warning: Unsupported declare 'foo' in e_compile_warning.php on line 6

Hello World

array(4) {
  ["type"]=>
  int(128)
  ["message"]=>
  string(25) "Unsupported declare 'foo'"
  ["file"]=>
  string(124) "e_compile_warning.php"
  ["line"]=>
  int(6)
}

Notice how the warning is displayed before the other output (even though "Hello World" came first in the source), and the var_dump statement on line 5 references an error that occurs on line 6. PHP compiles the script, doesn't like declare(foo='bar');, but goes back and executes the script anyway (as opposed to an E_COMPILE_ERROR like $this = 2;, which would stop execution (and compilation) immediately).

Hope this helps!

Answer 2

You can produce the output warning directly using the following code:

$cmd = "declare(foo='bar');";
eval($cmd);