Add separate checks for all polyfilled functions and constants

[JanJakes]

As originally discussed in symfony/polyfill-mbstring#6 and later in #251 in some scenarios it may be better to check for each constant/function before defining it. Such scenarios are typically those where some project or a library defines its polyfills for some parts of extension functionality but not all of it.

Probably the most popular example is WordPress, defining some polyfills in wp-includes/compat.php but not including complete functionalities (i.e. it defines only a few mb_ functions). Plugin developers then may need other mb_ functions and need to load those that WordPress didn't.

There may be some performance concerns but it seems that checks such as function_exists are very fast - in fact, PHP tries to evaluate both function_exists and defined at compile time (see also StackOverflow). This means on installs that do have the polyfilled extensions installed, there should be zero overhead (the conditions will evaluate to true at compile time).

Note that many of the currently existing polyfills already perform checks for each function separately (most of the PHP-version polyfills).

Closes #251.

[nicolas-grekas]

To validate the change, can you please run a simple benchmark with nothing else but a require for vendor/autoload.php, in three situations:

• all extensions loaded in php, polyfills installed
• no extensions loaded in php, polyfills installed
• again but with NO polyfills installed
  ?

If you think about other relevant benchmarks, please proceed :) Thanks.

[capuderg]

I've run some basic time benchmarks. I've only tested the mbstrings polyfill library (not this whole library)

A new PHP 7.4.5 CLI docker container was used as an environment.

I've run the basic require_once __DIR__ . '/vendor/autoload.php'; 1000 times and here are the averages (in seconds) for each situation:

• "empty" composer (no packages required) with mbstring extension disabled: 0.0179128654

• "empty" composer (no packages required) with mbstring extension enabled: 0.01777232742

• "symfony/polyfill-mbstring = 1.15" required with mbstring extension disabled: 0.02518279123

• "symfony/polyfill-mbstring = 1.15" required with mbstring extension enabled: 0.02492605114

• "symfony/polyfill-mbstring with updated bootstrap.php" required with mbstring extension disabled: 0.02665189552

• "symfony/polyfill-mbstring with updated bootstrap.php" required with mbstring extension enabled: 0.02570387244

I hope this helps :)

[JanJakes]

@capuderg Great, thanks! I didn't have time to get to this yet, only started to prepare a PHP Docker container without mbstring. This looks good, not sure if we need any more tests.

@nicolas-grekas Do you have any clues why the builds are failing? The Parse error: syntax error, unexpected T_OBJECT_OPERATOR in C:\projects\polyfill\vendor\bin\.phpunit\phpunit-4.8\phpunit on line 13 doesn't say much.

[nicolas-grekas]

Thank you @JanJakes.

[nicolas-grekas]

And thank you @capuderg also.