Essential Books

Check if key exists

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Use array_key_exists() or isset() or !empty(): 

$map = [
    'foo' => 1,
    'bar' => null,
    'foobar' => '',
];

array_key_exists('foo', $map); // true
isset($map['foo']); // true
!empty($map['foo']); // true

array_key_exists('bar', $map); // true
isset($map['bar']); // false
!empty($map['bar']); // false

Note that isset() treats a null valued element as non-existent. Whereas !empty() does the same for any element that equals false (using a weak comparision; for example, null, '' and 0 are all treated as false by !empty()). While isset($map['foobar']); is true, !empty($map['foobar']) is false. This can lead to mistakes (for example, it is easy to forget that the string '0' is treated as false) so use of !empty() is often frowned upon.

Note also that isset() and !empty() will work (and return false) if $map is not defined at all. This makes them somewhat error-prone to use: 

// Note "long" vs "lang", a tiny typo in the variable name.
$my_array_with_a_long_name = ['foo' => true];
array_key_exists('foo', $my_array_with_a_lang_name); // shows a warning
isset($my_array_with_a_lang_name['foo']); // returns false

You can also check for ordinal arrays: 

$ord = ['a', 'b']; // equivalent to [0 => 'a', 1 => 'b']

array_key_exists(0, $ord); // true
array_key_exists(2, $ord); // false

Note that isset() has better performance than array_key_exists() as the latter is a function and the former a language construct.

You can also use key_exists(), which is an alias for array_key_exists().

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Arrays:
* Arrays
* Initializing an Array
* Applying a function to each element of an array
* Check if key exists
* Validating the array type
* Iterating multiple arrays together
* Filtering an array
* Removing elements from an array
* Creating an array of variables
* Checking if a value exists in array
* Sorting an Array
* Splitting a string by separators
* Array intersection
* Adding element to start of array
* Merge or concatenate arrays
* ArrayAccess and Iterator Interfaces
* Whitelist only some array keys
* Imploding an array into string
* Split array into chunks
* Exchange values with keys
* Combining two arrays keys from one values from another
* array reduce
* Destructuring arrays using list
Table Of Contents
0 Getting started
1 Variables
2 Arrays
3 Functional programming
4 Types
5 Autoloading primer
6 Exception handling and error reporting
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35 UTF-8
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37 Object serialization
38 PHPDoc
39 Contributing to PHP Manual
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46 Type juggling and non-strict comparison issues
47 Security
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63 Contributing to PHP Core
64 Coding conventions
65 Using MongoDB
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68 Unicode Support
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74 Multiprocessing
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83 Sockets
84 Outputting the value of a variable
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88 Manipulating an array
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105 PHP mysqli
106 PHP built-in server
107 How to break down an URL
108 PSR
109 Contributors