Super simple strategy to pivot table like data

PHPivot.php

class Pivot {

  protected $pivotedData;

  public function __construct($data){
    $this->pivotedData = $this->_pivot($data);
  }

  protected function pivot($data){
    $pivoted = [];
    foreach ($data as $key => $values) {
      foreach ($values as $v) {
        if (!isset($pivoted[$v])) {
          $pivoted[$v] = array();
        }
        $pivoted[$][] = $key;
      }
    }
    
    return $pivoted;
  }

  public function getPivotedData() {
    return $this->pivotedData;
  }
} 

// a simple test ...
$data = [
  'x' => ['a', 'b', 'c'],
  'y' => ['a', 'c', 'd'],
];

$x = new Pivot($data);
print_r($x->getPivotedData());