Php array_rand() printing variable name

I have an array that is filled with different sayings and am trying to output a random one of the sayings. My program prints out the random saying, but sometimes it prints out the variable name that is assigned to the saying instead of the actual saying and I am not sure why.

$foo=Array('saying1', 'saying2', 'saying3');
$foo['saying1'] = "Hello.";
$foo['saying2'] = "World.";
$foo['saying3'] = "Goodbye.";

echo $foo[array_rand($foo)];

So for example it will print World as it should, but other times it will print saying2. Not sure what I am doing wrong.