-=+= What does it mean?

  question, vim

set dictionary-=/etc/vim/php_funclist.txt dictionary+=/etc/vim/php_funclist.txt

What are the meanings of-=+= respectively?

-= refers to the original value minus/etc/vim /etc/vim/php_funclist.txt
+= refers to the original code plus/etc/vim /etc/vim/php_funclist.txt
The function is equivalent to changing the value of dictionary to the constant path/etc/vim/PHP _ funcllist.txt.

Why not just write a sentence?
set dictionary+=/etc/vim/php_funclist.txt

`The use of |:set+=| and |:set-=| is preferred when adding or removing
 directories from the list.  This avoids problems when a future version
 uses another default.`

This sentence is incomprehensible and is used to avoid problems when future versions use other default values.

How can the path remain the same once you subtract and add? I just wrote dictionary+=/etc/vim/PHP _ funcllist.txt, the effect is the same, huh? ?

-= refers to the original value minus/etc/vim /etc/vim/php_funclist.txt
+= refers to the original code plus/etc/vim /etc/vim/php_funclist.txt
The function is equivalent to changing the value of dictionary to the constant path/etc/vim/PHP _ funcllist.txt.