PHP is difficult to understand

PHP version: PHP5 >= PHP5.2.0
Namespace is a way of encapsulating things,
Use namespace: relative file path, current working path relative to script path;
: relative subdirectory path, current working path relative to subdirectory path ;
: Jedi path: namespacedirectorytofile

empty() function: Determine whether the variable is considered empty: the condition for determining it to be empty is that the variable does not exist or the value of the variable is false;
The expression used is !isset($var) || $var==false;
Before PHP5.5, only variable names were supported, expressions were not supported,
Therefore empty(trim(var)) is grammatically wrong;
isset($var): Determination Whether the variable is set and the set value is not NULL;

The above has introduced the difficult aspects of PHP, including various aspects. I hope it will be helpful to friends who are interested in PHP tutorials.