What does ( !'which npm' ) mean in a PHP script?

问题

What does ( !'which npm' ) mean in the following code?

if ( !'which npm' ) {
  die(
        'You need to install NPM!' . PHP_EOL
  );
}

Here is a link to the complete file from GITHUB: WPBP/generator/bin/wpbp-generator

回答1:

It checks whether a hardcoded string is falsey. Which will always be false.

This logic looks a bit meaningless. I suspect the author rather wanted to write:

if (!`which npm`)

The backtick operator executes the command as shell command, which makes a bit more sense.

来源：https://stackoverflow.com/questions/52094786/what-does-which-npm-mean-in-a-php-script