Description:
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When you use array_key_exists() on a referenced array, it performs extremely slow compared to isset():

isset time 0.0053699016571045 size 0
isset time 0.0051219463348389 size 10
isset time 0.0032069683074951 size 100
isset time 0.0034000873565674 size 1000
isset time 0.0052909851074219 size 10000
array_key_exists time 0.0055620670318604 size 0
array_key_exists time 0.010941982269287 size 10
array_key_exists time 0.048230886459351 size 100
array_key_exists time 0.38906383514404 size 1000
array_key_exists time 5.6351599693298 size 10000

Due to the time increasing near linearly with the number of elements in the array, I assume the code for some reason looks through the array key linearly instead of using a hash lookup.

Test script:
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http://pastebin.com/PVrf96an