PHP array performance checking key existence vs checking values existence

Question

Is there a difference in O(n) of the below operations?

$a1 = [1=>'',2=>'', 3=>'']
isset($a1[2])

$a2 = [1,2, 3]
in_array(2, $a2)

in_array for values... sooo... they're not really meant for the same thing. — gloomy.penguin
– gloomy.penguin, Commented Oct 25, 2014 at 2:01
this is my guess... isset is faster because in_array() must look at the values. isset just wants to know if that index exists regardless of what it actually holds. — gloomy.penguin
– gloomy.penguin, Commented Oct 25, 2014 at 2:02

Antoan Milkov · Accepted Answer · 2014-10-25 07:58:44Z

2

isset($a1[2]) has complexity of O(1)
in_array(2, $a2) has complexity of O(3) in your case or in general O(N) where N=count_of_the_array elements

answered Oct 25, 2014 at 7:58

Antoan Milkov

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V. Bozz May 4 at 20:52

Thanks for this answer that is still incredibly useful in 2025. Maybe useful to add an external reference (documentation?) about this statement. I cannot find it right now, so I just comment here as community TODO

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