Tutorial :Variable Variables Pointing to Arrays or Nested Objects



Question:

Is it possible to create a variable variable pointing to an array or to nested objects? The php docs specifically say you cannot point to SuperGlobals but its unclear (to me at least) if this applies to arrays in general.

Here is my try at the array var var.

     // Array Example       $arrayTest = array('value0', 'value1');       ${arrayVarTest} = 'arrayTest[1]';       // This returns the correct 'value1'       echo $arrayTest[1];       // This returns null       echo ${$arrayVarTest};     

Here is some simple code to show what I mean by object var var.

     ${OBJVarVar} = 'classObj->obj';        // This should return the values of $classObj->obj but it will return null         var_dump(${$OBJVarVar});      

Am I missing something obvious here?


Solution:1

Array element approach:

  • Extract array name from the string and store it in $arrayName.
  • Extract array index from the string and store it in $arrayIndex.
  • Parse them correctly instead of as a whole.

The code:

$arrayTest  = array('value0', 'value1');  $variableArrayElement = 'arrayTest[1]';  $arrayName  = substr($variableArrayElement,0,strpos($variableArrayElement,'['));  $arrayIndex = preg_replace('/[^\d\s]/', '',$variableArrayElement);    // This returns the correct 'value1'  echo ${$arrayName}[$arrayIndex];  

Object properties approach:

  • Explode the string containing the class and property you want to access by its delimiter (->).
  • Assign those two variables to $class and $property.
  • Parse them separately instead of as a whole on var_dump()

The code:

$variableObjectProperty = "classObj->obj";  list($class,$property)  = explode("->",$variableObjectProperty);    // This now return the values of $classObj->obj  var_dump(${$class}->{$property});      

It works!


Solution:2

Use = & to assign by reference:

 $arrayTest = array('value0', 'value1');   $arrayVarTest = &$arrayTest[1];     $arrayTest[1] = 'newvalue1'; // to test if it's really passed by reference     print $arrayVarTest;  


Solution:3

In echo $arrayTest[1]; the vars name is $arrayTest with an array index of 1, and not $arrayTest[1]. The brackets are PHP "keywords". Same with the method notation and the -> operator. So you'll need to split up.

// bla[1]  $arr = 'bla';  $idx = 1;  echo $arr[$idx];    // foo->bar  $obj = 'foo';  $method = 'bar';  echo $obj->$method;  

What you want to do sounds more like evaluating PHP code (eval()). But remember: eval is evil. ;-)


Solution:4

Nope you can't do that. You can only do that with variable, object and function names.

Example:

 $objvar = 'classObj';   var_dump(${$OBJVarVar}->var);  

Alternatives can be via eval() or by doing pre-processing.

$arrayTest = array('value0', 'value1');  $arrayVarTest = 'arrayTest[1]';    echo eval('return $'.$arrayVarTest.';');  eval('echo $'.$arrayVarTest.';');  

That is if you're very sure of what's going to be the input.

By pre-processing:

function varvar($str){    if(strpos($str,'->') !== false){      $parts = explode('->',$str);      global ${$parts[0]};      return $parts[0]->$parts[1];    }elseif(strpos($str,'[') !== false && strpos($str,']') !== false){      $parts = explode('[',$str);      global ${$parts[0]};      $parts[1] = substr($parts[1],0,strlen($parts[1])-1);      return ${$parts[0]}[$parts[1]];    }else{      return false;    }  }    $arrayTest = array('value0', 'value1');  $test = 'arrayTest[1]';  echo varvar($test);  

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