Variables: overwriting vs if/else

This is a PHP question but is probably applicable to other languages.

Which is a better way to handle variable assignments?

// Use default value and overwrite
$var_one = 'x';
if ($var_two) {
    $var_one = 'y';
}

// Use complete if/else
if ($var_two) {
    $var_one = 'y';
} else {
    $var_one = 'x';
}

I'm always curious about how software works at the lowest levels.