What is the problem with the code below? What will it output? How can it be fixed?

$referenceTable = array();

$referenceTable['val1'] = array(1,2);

$referenceTable['val2'] =3;

$referenceTable['val3'] = array(4,5);

$testArray = array();

$testArray = array_merge($testArray, $referenceTable['val1']);

var_dump($testArray);

$testArray = array_merge($testArray, $referenceTable['val2']);

var_dump($testArray);

$testArray = array_merge($testArray, $referenceTable['val3']);

var_dump($testArray);