What is the error in the script?

Question

PHP

All OK:

$a = 'вывод';
eval("echo \"$a\";");

So

$a = 'вывод';
eval("echo $a;");

Notice: Use of undefined constant output - assumed 'output' in ...test.php(5) : eval()'d code on line 1
output
Where does he see the constant? Ok, let's do:

$a = '\'вывод\'';
eval("echo \"$a\";");

On screen 'output' (together with quotes).
So in PHP it should turn into like and then the word without quotes should be shown on the screen? echo 'вывод';