A function that traverses file directories. I have two questions. Could you please give me some advice?

Question 1: The static of static $file_list is used for recursive superposition. How to avoid more than two external calls, and the results of the previous calls are superimposed on the subsequent results?
Question 2: Is it reasonable to divide the formal parameter $ext_name into two parameters ($mode=true, $ext_name='')?

//Traverse files (file list)
static function scan($dir,$ext_name=true){

$dir_tree=array();
static $file_list=null;
static $the_file_list=null;

foreach (scandir($dir) as $file) {

$file_location=trim($dir,'/').'/'.$file;

if (is_dir($file_location) && $file!=" ." && $file!="..") {
array_push($dir_tree,self::scan($file_location,$ext_name));
} else {

if (($file!='.' && $ file!='..') || !is_dir($file)) {
if ($ext_name===true) {
$file_list[]=$file_location;
}

if ($ext_name && $ext_name= =trim(strrchr($file,'.'))) {
$the_file_list[]=$file_location;
}

$tmp=explode('/',$dir);
array_push($dir_tree,'< strong>'.end($tmp).' '.$file);
}

}

}

$result=($ext_name===true) ? $file_list : $the_file_list;
return $ext_name ? $result : $dir_tree;
}