gistfile1.php

$data = array(
  'firstname' => array('op' => 'eq', 'value' => 'Stephan'),
  'username' => array('op' => 'contains', 'value' => 'swe'),
);

// transform data to expression

$expr = Expr::valid();

foreach ($data as $field => $match) {
  if ($match['op'] === 'eq') {
    $expr = $expr->andEquals($match['value']);
  } elseif ($match['op'] === 'contains') {
    $expr = $expr->andContains($match['value']);
  }
}

// evaluate criteria ...