Associative array creation in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an associative array in PHP?

An associative array in PHP is a collection of key-value pairs where each key is a unique string or integer used to access its corresponding value.

beginner

How do you create an associative array with keys 'name' and 'age' in PHP?

You create it like this:
$person = ['name' => 'Alice', 'age' => 30];
This means 'name' points to 'Alice' and 'age' points to 30.

intermediate

What happens if you use the same key twice in an associative array?

The last value assigned to that key will overwrite any previous value. Keys must be unique in an associative array.

beginner

How can you access the value of the key 'age' in an associative array named $person?

Use the syntax: $person['age']; This will give you the value stored under the key 'age'.

beginner

Can keys in PHP associative arrays be integers?

Yes, keys can be integers or strings. PHP treats integer keys as normal keys in associative arrays.

Which of the following is the correct way to create an associative array in PHP?

A $arr = array('red', 'large');

B $arr = ('color', 'red', 'size', 'large');

C $arr = ['red', 'large'];

D $arr = ['color' => 'red', 'size' => 'large'];

Option D uses the correct syntax with keys and values. Options A, B, and C do not create associative arrays with keys.

What will be the output of echo $arr['name']; if $arr = ['name' => 'Bob'];?

A Bob

B name

C Error

D Array

Accessing $arr['name'] returns the value 'Bob' stored under the key 'name'.

If you create $arr = ['key' => 'first', 'key' => 'second'];, what is the value of $arr['key']?

A first

B second

C Array

D Error

The second value overwrites the first for the same key, so $arr['key'] is 'second'.

Which data types can be used as keys in PHP associative arrays?

A Only strings

B Only integers

C Strings and integers

D Any data type

PHP allows keys to be strings or integers in associative arrays.

How do you add a new key-value pair 'city' => 'Paris' to an existing associative array $arr?

A $arr['city'] = 'Paris';

B $arr->city = 'Paris';

C array_push($arr, 'city' => 'Paris');

D $arr.add('city', 'Paris');

You add a new key-value pair by assigning it directly: $arr['city'] = 'Paris';

Explain how to create and access values in an associative array in PHP.

Think about how you label things with names and then find them by those names.

Describe what happens if you use duplicate keys when creating an associative array.

Imagine writing a label twice on the same box.

Practice

1. Which of the following is the correct way to create an associative array in PHP with keys "name" and "age"?

easy

A. $person = ("name" => "Alice", "age" => 30);

B. $person = {"name": "Alice", "age": 30};

C. $person = array("Alice", 30);

D. $person = ["name" => "Alice", "age" => 30];

Solution

1. Step 1: Understand PHP associative array syntax

   Associative arrays use square brackets with key => value pairs.

2. Step 2: Identify correct syntax among options

   $person = ["name" => "Alice", "age" => 30]; uses square brackets and => correctly; others use invalid syntax.

3. Final Answer:

   $person = ["name" => "Alice", "age" => 30]; -> Option D

4. Quick Check:

   Associative array = key => value pairs [OK]

Hint: Use square brackets and => for associative arrays [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using colon (:) instead of =>
• Creating indexed arrays instead of associative

2. Which of the following PHP code snippets will cause a syntax error when creating an associative array?

easy

A. $data = ["key1" : "value1", "key2" : "value2"];

B. $data = array("key1" => "value1", "key2" => "value2");

C. $data = ["key1" => "value1", "key2" => "value2"];

D. $data = ['key1' => 'value1', 'key2' => 'value2'];

Solution

1. Step 1: Check the syntax for key-value pairs

   PHP uses => to assign values to keys, not colon (:).

2. Step 2: Identify the invalid syntax

   $data = ["key1" : "value1", "key2" : "value2"]; uses colon (:) which is invalid in PHP associative arrays.

3. Final Answer:

   $data = ["key1" : "value1", "key2" : "value2"]; -> Option A

4. Quick Check:

   Use => not : for keys in PHP arrays [OK]

Hint: Remember PHP uses =>, not : for associative arrays [OK]

Common Mistakes:

• Using colon (:) instead of =>
• Mixing array() and [] syntax incorrectly
• Missing commas between pairs

3. What will be the output of the following PHP code?

$arr = ["fruit" => "apple", "color" => "red"];
echo $arr["color"];

medium

A. red

B. color

C. apple

D. fruit

Solution

1. Step 1: Understand array key access

   Accessing $arr["color"] returns the value for key "color".

2. Step 2: Identify the value for key "color"

   Value is "red", so echo outputs "red".

3. Final Answer:

   red -> Option A

4. Quick Check:

   Key "color" = "red" [OK]

Hint: Access values by keys with $array["key"] [OK]

Common Mistakes:

• Confusing keys and values
• Using wrong quotes or missing quotes
• Trying to access with dot notation

4. Identify the error in the following PHP code that tries to create an associative array:

$info = ["name" => "John", "age" = 25];

medium

A. Missing comma between elements

B. Keys should not be in quotes

C. Using = instead of => for key-value pair

D. Array must use array() syntax

Solution

1. Step 1: Check key-value assignment syntax

   PHP requires => to assign values to keys, not =.

2. Step 2: Locate incorrect assignment

   "age" = 25 uses = instead of =>, causing syntax error.

3. Final Answer:

   Using = instead of => for key-value pair -> Option C

4. Quick Check:

   Use => for key-value pairs [OK]

Hint: Use =>, not =, between key and value [OK]

Common Mistakes:

• Using single = instead of =>
• Omitting commas between pairs
• Not quoting string keys

5. You want to create an associative array in PHP that maps product names to their prices, but only include products with a price greater than 20. Which code correctly creates this filtered associative array?

hard

A. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $name, $price) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

B. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $name => $price) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

C. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = array_filter($products, function($name, $price) {
return $price > 20;
});

D. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $price => $name) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

Solution

1. Step 1: Understand foreach key-value syntax

   Use foreach ($array as $key => $value) to access keys and values correctly.

2. Step 2: Filter products with price > 20

   $products = ["apple" => 15, "banana" => 25, "cherry" => 30]; $filtered = []; foreach ($products as $name => $price) { if ($price > 20) { $filtered[$name] = $price; } } correctly checks price and assigns filtered items to new array.

3. Final Answer:

   The code using foreach ($products as $name => $price) -> Option B

4. Quick Check:

   foreach ($arr as $key => $value) for associative arrays [OK]

Hint: Use foreach ($arr as $key => $value) to access associative arrays [OK]

Common Mistakes:

• Swapping key and value variables in foreach
• Using incorrect foreach syntax with two variables separated by comma
• Misusing array_filter with wrong callback parameters