Php get array elememnt with $$

Your statement ( $$target_arr[5]

) is ambiguous. PHP doesn't know what you actually want to say: do you mean: use a $target_arr[5]

value and add $

to use that as a variable, or do you want to use a value $target_arr

and get the fifth element of that array?
Obviously this is the latter, but PHP doesn't know this. To eliminate your expression, you must use curly braces:

${$target_arr}[5];

      

        
        
        
      

    

This will give 10. See the manual on variable variables for details

Note:
As people say in the comments and deleted answers: variable variables like the one you use is risky business. 9/10 this can and should be avoided. This makes your code harder to read, more error-prone, and when combined with these two major flaws is a killer: it makes your code incredibly hard to debug .
If this is just a technical exercise, consider this post as friendly advice. If you got this from some sort of tutorial / blog or other type of online resource: never visit this site again.
If you are actually working on a piece of code and you have decided to solve a specific problem with the vars variables, then perhaps post your code in the code overviewand let me know, I will take a look and try to offer constructive criticism that will help you on your way to a better solution.

Since what you are actually trying to do is copy the array into another variable, this is pretty straightforward. PHP offers many ways to do this:

Copy on assignment:
PHP copies arrays on default assignment, so this means that:

$someArray = range(1,10);//[1,2,3,4,5,6,7,8,9,10]
$foo = $someArray;

      

        
        
        
      

    

Assigns a copy to the $someArray

variable $foo

:

echo $foo[0], ' === ', $someArray[0];//echoes 1 === 1
$foo[0] += 123;
echo $foo[0], ' != ', $someArray[0];//echoes 123 != 1

      

        
        
        
      

    

I can change the value of one of the elements in the array without affecting the original array because it was copied.
There is a risk for this, since you start out with JSON encoded data, chances are you will end up with something like:

$obj = json_decode($string);
echo get_class($obj));//echoes stdClass, you have an object

      

        
        
        
      

    

Objects are passed and assigned by reference by default, which means that:

$obj = new stdClass;
$obj->some_property = 'foobar';
$foo = $obj;
$foo->some_property .= '2';
echo $obj->some_property;//echoes foobar2!

      

        
        
        
      

    

Change the property via $foo

and the object $obj

will change too. Simply because they both refer to the same object.

Slice Array:
A more general way for frontend developers (basically stemming from the JS habbit I think) is to use array_slice

that guarantees a copy of the array is returned. with the added perk that you can specify how many of the items you need in your copy:

$someArray = range(1,100);//"large" array
$foo = array_slice($someArray, 0);//copy from index 0 to the end
$bar = array_slice($someArray, -10);//copy last 10 elements
$chunk = array_slice($someArray, 20, 4);//start at index 20, copy 4 elements

      

        
        
        
      

    

If you don't want to copy the array, but rather extract the slice from the original, you can splice

array (as in split + slice):

$extract = array_splice($someArray, 0, 10);
echo count($someArray);//echoes 90

      

        
        
        
      

    

This removes the first 10 elements from the original array and assigns them $extract

Spend some time browsing the countless (well, about a hundred) array functions PHP has to offer.