String Evaluates to 0

gistfile1.txt

<?php

$arr = array();
$arr['foo'] = 'bar';

// 'boo' evaluates to 0
echo $arr['foo']['boo']; // b

// This evaluates as true and echos
if (isset($arr['foo']['boo'])) {
	echo 'FooBoo'; 
}

// What was intended. Does not echo
if (isset($arr['foo']) && is_array($arr['foo']) && isset($arr['foo']['boo'])) {
	echo 'This is what I really meant';
}

// Evaluates as false, '10' treated as its numeric value, does not echo
if (isset($arr['foo']['10'])) {
	echo 'FooBoo2';
}

// Evaluates as true, '1' treated as its numeric value, echos
if (isset($arr['foo']['1'])) {
	echo $arr['foo']['1']; //a (second character in 'bar')
}

?>