[php] How to check if an array element exists?

Example: I'm checking for the existence of an array element like this:

if (!self::$instances[$instanceKey]) {
    $instances[$instanceKey] = $theInstance;
}

However, I keep getting this error:

Notice: Undefined index: test in /Applications/MAMP/htdocs/mysite/MyClass.php on line 16

Of course, the first time I want an instance, $instances will not know the key. I guess my check for available instance is wrong?

array_key_exists() is SLOW compared to isset(). A combination of these two (see below code) would help.

It takes the performance advantage of isset() while maintaining the correct checking result (i.e. return TRUE even when the array element is NULL)

if (isset($a['element']) || array_key_exists('element', $a)) {
       //the element exists in the array. write your code here.
}

The benchmarking comparison: (extracted from below blog posts).

array_key_exists() only : 205 ms
isset() only : 35ms
isset() || array_key_exists() : 48ms

See http://thinkofdev.com/php-fast-way-to-determine-a-key-elements-existance-in-an-array/ and http://thinkofdev.com/php-isset-and-multi-dimentional-array/

for detailed discussion.

You can use isset() for this very thing.

$myArr = array("Name" => "Jonathan");
print (isset($myArr["Name"])) ? "Exists" : "Doesn't Exist" ;

A little anecdote to illustrate the use of array_key_exists.

// A programmer walked through the parking lot in search of his car
// When he neared it, he reached for his pocket to grab his array of keys
$keyChain = array(
    'office-door' => unlockOffice(),
    'home-key' => unlockSmallApartment(),
    'wifes-mercedes' => unusedKeyAfterDivorce(),
    'safety-deposit-box' => uselessKeyForEmptyBox(),
    'rusto-old-car' => unlockOldBarrel(),
);

// He tried and tried but couldn't find the right key for his car
// And so he wondered if he had the right key with him.
// To determine this he used array_key_exists
if (array_key_exists('rusty-old-car', $keyChain)) {
    print('Its on the chain.');
}

According to the php manual you can do this in two ways. It depends what you need to check.

If you want to check if the given key or index exists in the array use array_key_exists

<?php
$search_array = array('first' => 1, 'second' => 4);
if (array_key_exists('first', $search_array)) {
echo "The 'first' element is in the array";
 }
?>

If you want to check if a value exists in an array use in_array

 <?php
 $os = array("Mac", "NT", "Irix", "Linux");
if (in_array("Irix", $os)) {
echo "Got Irix";
}
?>

You can also use array_keys for number of occurrences

<?php
$array=array('1','2','6','6','6','5');
$i=count(array_keys($array, 6));
if($i>0)
 echo "Element exists in Array";
?>

You want to use the array_key_exists function.

You can use the function array_key_exists to do that.

For example,

$a=array("a"=>"Dog","b"=>"Cat");
if (array_key_exists("a",$a))
  {
  echo "Key exists!";
  }
else
  {
  echo "Key does not exist!";
  }

PS : Example taken from here.