Arn, 3 bytes

phi 

Just a builtin, too short to be compressed. Returns 1.61803398874989484820458683436563811.

A more interesting one (returns 1.61803398874989484821):

6 bytes

l[├Qn0 

Try it!

Explanation

Unpacked: :-1+:/5

:- Halve 1 Literal one + Plus :/ Square root of 5 Literal five 

Arn, 3 bytes

phi 

Just a builtin, too short to be compressed. Returns 1.61803398874989484820458683436563811.

A more interesting one. If running in the downloadable version, this will print n digits, given the command:

arn run file.arn -p n 

or 25 digits if the -p flag is not provided.

If running in the online version, this will print the first 50 digits.

6 bytes

l[├Qn0 

Try it!

Explanation

Unpacked: :-1+:/5

:- Halve 1 Literal one + Plus :/ Square root of 5 Literal five