$content variable – Is this a reserved variable for a WordPress function? – php / wordpress

$content – is not reserved in WordPress, it can be used.

In the function the_content () – a variable is used, but it is only available in this function.
The code of the function itself:

https://core.trac.wordpress.org/browser/tags/4.7/src/wp-includes/post-template.php#L230

Scope of variables in php:

http://php.net/manual/en/language.variables.scope.php