How does the core 'php.ini' directive 'memory_limit' is considered as an 'Environment Variable' in an code example from the PHP Manual?

Question

I'm using PHP 7.2.8 on my machine that runs on Windows 10 64-bit Operating System .

I come across the following code example from PHP Manual having some description :

Using environment variables can be used in php.ini as shown below.

Example #1 php.ini Environment Variables

 ; PHP_MEMORY_LIMIT is taken from environment memory_limit = ${PHP_MEMORY_LIMIT} 

I executed the above code on my machine it gave me the following error messages :

Warning: Use of undefined constant PHP_MEMORY_LIMIT - assumed 'PHP_MEMORY_LIMIT' (this will throw an Error in a future version of PHP) in demo.php on line 3

Notice: Undefined variable: PHP_MEMORY_LIMIT in demo.php on line 3

I checked the list of available 'Environment Variables' on my machine but I didn't get such environment variable named $_ENV['PHP_MEMORY_LIMIT']

Then, I checked the php.ini file present on my machine. I found the core php_directive memory_limit=128M

So, my question is why the code example from PHP Manual is considering something similar( ${PHP_MEMORY_LIMIT} ) to the core 'php.ini' directive as an 'Environment Variable' ?

Also, the code written in this code example doesn't work as such environment variable named ${PHP_MEMORY_LIMIT} doesn't exist.

I even checked the latest php.ini from Git as recommended in the PHP Manual but in that file also I found the core php_directive memory_limit=128M and no environment similar to it.

Is the PHP Manual having wrong code example?

Someone please clear my doubts.

Thanks.

Answer 1

The first thing to note is that your example is not a code example, it's a configuration example. So it belongs on php.ini and not on a PHP file.

Environment variables are system dependent and it is up to you to set them. The manual is giving you an example with the aptly named PHP_MEMORY_LIMIT . You could set that variable on your system and then use it on your php.ini . But you can call it whatever you want, that's just an example, it's not based on any specific platform where that variable would be set.