PHP Check if static variable has been declared or initialized?

Question

I want to check if a static variable has been declared/initialized previously, e.g. if a function with a static variable is being run for the first time. See the following example code:

function init_i (){
    // check if $i is set??
    if(isset($i)) echo '$i is static and is set ';


    static $i=0;
    $i++;
    echo "$i<br>";
}

function run_init(){
    init_i();
}

run_init(); //output 1
run_init(); //should output $i is static and is set 2
run_init(); //should output $i is static and is set 3
run_init(); //should output $i is static and is set 4
run_init(); //should output $i is static and is set 5

The problem is that isset($i) never seems to prove true even though it is a static variable. How do I check static $i has already been set?

Answer 1

Just omit out the default value, and it will be null:

static $i;

// check if $i is set??
if(isset($i)){
  echo '$i is static and is set ';
}else{
  // first call, initialize...
  $i = 0;
}

...

isset() returns TRUE if variable is set and not null.

I don't get what's your reasoning behind this, because you can just check the value is the initial value (0) and you know that's the first call...