unset variable in php
Let's consider the function:
function foo() { static $bar; $bar++; unset($bar);}foo(); //static $bar is 1foo(); //static $bar is 2
The function compiles to:
function name: foonumber of ops: 11compiled vars: !0 = $barline # * op fetch ext return operands--------------------------------------------------------------------------------- 2 0 > EXT_NOP 4 1 EXT_STMT 2 FETCH_W static $0 'bar' 3 ASSIGN_REF !0, $0 5 4 EXT_STMT 5 POST_INC ~1 !0 6 FREE ~1 6 7 EXT_STMT 8 UNSET_VAR !0 7 9 EXT_STMT 10 > RETURN null
A variable actually exists outside each function call to foo()
and, on each call, it's fetched and a reference to it is assigned to $bar
. In fact, it's very similar to this:
function foo() { global $bar; $bar++; unset($bar);}
When you call unset()
, you're only destroying the reference you created, not the underlying value.
I didn't confirm, but what I'd guess that happens is this:
- The underlying representation of the variabe (the zval) is stored so that its reference count is 1.
- When
foo()
is called, the symbol$bar
is associated with this zval, its reference count is increased to 2 and the reference flag is set. - When
unset
is called, the zval has its reference count decreased to 1, the reference flag is probably cleared and the symbol$bar
is removed.
unset(self::$somethingstatic); will raise an Fatal error, because the variable is static (always there, can't be unset).
the documentation refers specifically to static variables defined inside a function, consider:
function t($stage){ static $shell = 23; switch($stage) { case 1: $shell++; break; case 2: unset($shell); break; case 3: $shell--; break; } echo $shell;}
because $shell is a static variable, it's always there (static) so any other time you mention $shell that is simply a reference - when you unset it, you are unsetting the reference (think unlink'ing a symlink) - the static variable is however still there (that's what static means).
thus if you call the above function t(1) will echo 24, t(2) will echo nothing, and t(3) will (rightly) echo 23 :)
help any?