A bit of confusion about arrays-php Tutorial

PHPcode $ arrarray (); $ numhelloworld; if (isset ($ num) {$ arr $ num; var_dump ($ arr );} else {$ arr [] array (, 3); var_dump ($ arr);} after running, the browser displays a bit of confusion about arrays in string HTMLc.

PHP code

  $arr = array();$num='hello world';if(isset($num)) {    $arr = $num;    var_dump($arr);} else {    $arr[] = array(1,2,3);    var_dump($arr);}

After running, the browser displays the string type

HTML code

  string(11) "hello world" 

But I have already set the arr as an array. I don't understand the internal cause, or $ arr = $ num; the variable arr has been declared again ???

------ Solution --------------------
This is the weak type ......

$ Str = "123 ";
Echo $ str [0]; // no problem in this way. output 1
------ Solution --------------------
Variable types can be automatically changed with context, which is a so-called weak type.

You have $ arr = $ num, so $ arr becomes a string.
------ Solution --------------------
The PHP type changes at any time and requires forced type conversion, like this: var_dump (array) $ arr );