PHP Array - Turning Array values into Keys

Question

What is the MOST EFFICIENT way to have an array of values and turn it into an array of keys? I'd really like to avoid any foreach loop...

$in = array(
    'red',
    'green',
    'blue'
);

INTO

$out = array(
    'red' => NULL,
    'green' => NULL,
    'blue' => NULL
);

Answer 1

Use PHP's array_flip function.

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On second thought, if you want the values to be null, then you might want to use array_fill_keys:

$out = array_fill_keys($in, null);