<?php

function test( $test ) {
	$res = json_decode( $test );
	$err = $res === null ? ' (' . json_last_error_msg() . ')' : '';
	echo "$test => " . var_export( json_decode( $test ), 1 ) . "$err\n";
}

echo PHP_VERSION . "\n";

echo "\nPassing tests:\n";
test( '"true"' );
test( 'true' );
test( '"false"' );
test( 'false' );
test( '"null"' );
test( 'null' );

echo "\nShould fail:\n";
test( 'trueOBVIOUSLYnot' );
test( 'falseOBVIOUSLYnot' );
test( 'nullOBVIOUSLYnot' );
test( 'true, {} nil; [] false' );

$ php test-107132.php 
5.6.99-hhvm

Passing tests:
"true" => 'true'
true => true
"false" => 'false'
false => false
"null" => 'null'
null => NULL (No error)

Should fail:
trueOBVIOUSLYnot => true
falseOBVIOUSLYnot => false
nullOBVIOUSLYnot => NULL (No error)
true, {} nil; [] false => true

5.5.9-1ubuntu4.11

Passing tests:
"true" => 'true'
true => true
"false" => 'false'
false => false
"null" => 'null'
null => NULL (No error)

Should fail:
trueOBVIOUSLYnot => NULL (unexpected character)
falseOBVIOUSLYnot => NULL (unexpected character)
nullOBVIOUSLYnot => NULL (unexpected character)
true, {} nil; [] false => NULL (unexpected character)