What s the difference between isset and array key exists

Question

How do the following two function calls compare   isset  a  key     array key exists  key    a

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The PHP function array key exists   determines if a particular key  or numerical index  exists for an element of an array  However  if you want to determine if a key exists and is associated with a value  the PHP language construct isset   can tell you that  and that the value is not null   array key exists  cannot return information about the value of a key index

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Between array key exists and isset  though both are very fast  O 1    isset is significantly faster  If this check is happening many thousands of times  you d want to use isset  It should be noted that they are not identical  though -- when the array key exists but the value is null  isset will return false and array key exists will return true  If the value may be null  you need to use array key exists   As noted in comments  if your value may be null  the fast choice is  isset  foo  key      array key exists  key   foo

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Function isset   is faster  check http   www php net manual en function array-key-exists php 82867

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The two are not exactly the same  I couldn t remember the exact differences  but they are outlined very well in What  39 s quicker and better to determine if an array key exists in PHP    The common consensus seems to be to use isset whenever possible  because it is a language construct and therefore faster  However  the differences should be outlined above

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Answer to an old question as no answer here seem to address the  warning  problem  explanation follows   Basically  in this case of checking if a key exists in an array  isset   tells if the expression  array  is defined  and the key is set no warning or error if the var is not defined  not an array     but returns false if the value for that key is null   and array key exists   tells if a key exists in an array as the name implies but gives a warning if the array parameter is not an array   So how do we check if a key exists which value may be null in a variable   that may or may not be an array  or similarly is a multidimensional array for which the key check happens at dim 2 and dim 1 value may not be an array for the 1st dim  etc        without getting a warning  without missing the existing key when its value is null  what were the PHP devs thinking would also be an interesting question  but certainly not relevant on SO   And of course we don t want to use    isset  var  key                  silent but misses null values array key exists  key   var      works but warning if  var not defined array   It seems is array should be involved in the equation  but it gives a warning if  var is not defined  so that could be a solution   if  isset  var  key           isset  var   amp  amp  is array  var   amp  amp  array key exists  key   var         which is likely to be faster if the tests are mainly on non-null values  Otherwise for an array with mostly null values  if  isset  var   amp  amp  is array  var   amp  amp  array key exists  key   var         will do the work

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Complementing  as an algebraic curiosity  the   deceze answer with the   operator  and indicating cases where is  better  to use       Not really better  if you need  no log and  micro-performance optimization    array key exists  is true if a key exists in an array   isset  is true if the key variable exists and is not null  faster than array key exists       array  key    is true if the key variable exists and is not  null  or    or  0    so much slower       a   array  k1    gt   HELLO    k2    gt  null   k3    gt       k4    gt  0    print isset  a  k1      OK  a k1      NO VALUE                  OK print array key exists  k1    a    OK  a k1      NO VALUE       OK print   a  k1     OK  a k1      NO VALUE                        OK    outputs OK HELLO   OK HELLO  OK HELLO   print isset  a  k2      OK  a k2      NO VALUE                  NO print array key exists  k2    a    OK  a k2      NO VALUE       OK print   a  k2     OK  a k2      NO VALUE                        NO    outputs NO VALUE   OK    NO VALUE   print isset  a  k3      OK  a k3      NO VALUE                  OK print array key exists  k3    a    OK  a k3      NO VALUE       OK print   a  k3     OK  a k3      NO VALUE                        NO    outputs OK   OK   NO VALUE   print isset  a  k4      OK  a k4      NO VALUE                  OK print array key exists  k4    a    OK  a k4      NO VALUE       OK print   a  k4     OK  a k4      NO VALUE                        NO    outputs OK 0  OK 0  NO VALUE     PS  you can change correct complement this text  it is a Wiki

User · Answer

array key exists will definitely tell you if a key exists in an array  whereas isset will only return true if the key variable exists and is not null    a   array  key1    gt           key2    gt  null    isset  a  key1                    true array key exists  key1    a       true  isset  a  key2                    false array key exists  key2    a       true   There is another important difference  isset doesn t complain when  a does not exist  while array key exists does

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The main difference when working on arrays is that array key exists returns true when the value is null  while isset will return false when the array value is set to null   See isset on the PHP documentation site

[php] What's the difference between isset() and array_key_exists()?

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