Conjure Arrays from nothing

<?php

//$thevar starts undefined

$thevar["that"]=5;
var_dump($thevar);

// outputs
// array(1) {
//   ["that"]=>
//   int(5)
// }

$thevar=null;
var_dump($thevar);
$thevar["that"]=5;
var_dump($thevar);

// outputs
// NULL
// array(1) {
//   ["that"]=>
//   int(5)
// }

$thevar=false;
var_dump($thevar);
$thevar["that"]=5;
var_dump($thevar);

// outputs
// bool(false)
// array(1) {
//   ["that"]=>
//   int(5)
// }

// all code above outputs with no warnings

$thevar=true;
var_dump($thevar);
$thevar["that"]=5;
var_dump($thevar);

// outputs
// bool(true)
// stderr - PHP Warning:  Cannot use a scalar value as an array
// bool(true)