Php: if or else

Imagine, that you have a variable called $varName. It can take one of two values: 'aaa' or 'bbb'. So, which statement will be optimal and why?

1)

if ($varName === 'aaa') {
/* Code for 'aaa' */
} else {
/* Code for 'bbb' */
}

2)

if ($varName === 'bbb') {
/* Code for 'bbb' */
} else {
/* Code for 'aaa' */
}

3)

if ($varName == 'aaa') {
/* Code for 'aaa' */
} else {
/* Code for 'bbb' */
}

4)

if ($varName == 'bbb') {
/* Code for 'bbb' */
} else {
/* Code for 'aaa' */
}

UPD: Variable take 'aaa' value more ofter than 'bbb'