Compact and extract functions in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the compact() function do in PHP?

The compact() function creates an array from variables and their values. You give it variable names as strings, and it returns an associative array where keys are the variable names and values are the variable values.

beginner

What is the purpose of the extract() function in PHP?

The extract() function imports variables into the current symbol table from an associative array. It creates variables named after the array keys and assigns them the corresponding values.

intermediate

How do compact() and extract() relate to each other?

compact() turns variables into an array, while extract() turns an array back into variables. They are like opposites and can be used to pass variables around easily.

intermediate

What happens if you use extract() with an array that has keys matching existing variables?

By default, extract() will overwrite existing variables with the same name. You can control this behavior with flags like EXTR_SKIP to avoid overwriting.

beginner

Show a simple example of using compact() and extract() together.

Example:

$name = "Alice";
$age = 30;
$data = compact('name', 'age');
// $data is ['name' => 'Alice', 'age' => 30]
extract($data);
// Now $name and $age variables exist again with original values

What does compact('x', 'y') return if $x = 5 and $y = 10?

A Variables named 'x' and 'y' with values 5 and 10

B An array [5, 10]

C An array ['x' => 5, 'y' => 10]

D A string 'x y'

compact() returns an associative array with keys as variable names and values as their values.

What does extract(['a' => 1, 'b' => 2]) do?

A Deletes variables <code>$a</code> and <code>$b</code>

B Creates variables <code>$a = 1</code> and <code>$b = 2</code>

C Creates an array with keys 'a' and 'b'

D Returns an array [1, 2]

extract() creates variables named after the array keys with their corresponding values.

If you want to avoid overwriting existing variables when using extract(), which flag should you use?

A EXTR_OVERWRITE

B EXTR_IF_EXISTS

C EXTR_PREFIX_ALL

D EXTR_SKIP

EXTR_SKIP skips extracting variables if they already exist, preventing overwriting.

Which function would you use to turn variables into an associative array?

A compact()

B array_keys()

C array_merge()

D extract()

compact() creates an associative array from variables.

What will happen if you call extract() on an empty array?

A Nothing happens, no variables are created

B It throws an error

C It creates variables with null values

D It deletes all existing variables

If the array is empty, extract() does nothing and no variables are created.

Explain how compact() and extract() work and how they can be used together.

Think about turning variables into arrays and back.

What precautions should you take when using extract() in your code?

Consider variable conflicts and safety.

Practice

1. What does the PHP function compact do?

easy

A. Creates an array from given variable names and their values

B. Extracts variables from an array into the current symbol table

C. Deletes variables from the current scope

D. Converts an array into a JSON string

Solution

1. Step 1: Understand the purpose of compact

   The compact function takes variable names as strings and creates an associative array where keys are those names and values are the variables' values.

2. Step 2: Differentiate from extract

   extract does the opposite by creating variables from array keys, so it is not the correct description for compact.

3. Final Answer:

   Creates an array from given variable names and their values -> Option A

4. Quick Check:

   compact = array from variables [OK]

Hint: Remember: compact packs variables into an array [OK]

Common Mistakes:

• Confusing compact with extract
• Thinking compact extracts variables
• Assuming compact converts arrays to strings

2. Which of the following is the correct syntax to use extract on an array $data?

easy

A. extract = $data;

B. extract($data);

C. extract->$data();

D. extract($data[]);

Solution

1. Step 1: Recall the function call syntax

   In PHP, functions are called by their name followed by parentheses containing arguments. So extract($data); is the correct syntax.

2. Step 2: Identify incorrect syntax

   Options B, C, and D misuse assignment, object operator, or array syntax, which are invalid for function calls.

3. Final Answer:

   extract($data); -> Option B

4. Quick Check:

   Function call syntax = extract($data); [OK]

Hint: Use parentheses to call extract with array argument [OK]

Common Mistakes:

• Using assignment instead of function call
• Using object operator (->) incorrectly
• Adding brackets after variable name

3. What is the output of the following PHP code?

$name = "Alice";
$age = 30;
$arr = compact('name', 'age');
print_r($arr);

medium

A. Array ( [Alice] => name [30] => age )

B. Array ( [0] => name [1] => age )

C. Error: Undefined variables

D. Array ( [name] => Alice [age] => 30 )

Solution

1. Step 1: Understand compact behavior

   The compact function creates an array where keys are the variable names as strings and values are the current values of those variables.

2. Step 2: Apply to given variables

   Variables $name and $age have values "Alice" and 30, so the array will be ['name' => 'Alice', 'age' => 30].

3. Final Answer:

   Array ( [name] => Alice [age] => 30 ) -> Option D

4. Quick Check:

   compact('name','age') = array with keys and values [OK]

Hint: compact creates array with variable names as keys [OK]

Common Mistakes:

• Thinking compact returns variable values as keys
• Expecting numeric keys instead of variable names
• Assuming compact prints variables directly

4. Identify the error in this PHP code snippet:

$data = ['color' => 'red', 'size' => 'large'];
extract($data);
echo $colour;

medium

A. Variable name mismatch: $colour vs 'color' key

B. extract() cannot be used on associative arrays

C. Missing semicolon after extract()

D. echo cannot print variables created by extract()

Solution

1. Step 1: Check variable names created by extract()

   The extract function creates variables named exactly as the array keys, so it creates $color and $size.

2. Step 2: Compare echoed variable name

   The code tries to echo $colour which is different from $color, causing an undefined variable notice.

3. Final Answer:

   Variable name mismatch: $colour vs 'color' key -> Option A

4. Quick Check:

   extract keys = variable names; spelling must match [OK]

Hint: Check spelling of variable names after extract [OK]

Common Mistakes:

• Assuming extract changes variable names
• Thinking extract only works on indexed arrays
• Ignoring PHP notices for undefined variables

5. Given the array $info = ['name' => 'Bob', 'age' => 25, 'city' => 'Paris', 'age' => 30];, what will be the result of extract($info); echo $age;?

hard

A. 25

B. Error: Duplicate keys in array

C. 30

D. Undefined variable error

Solution

1. Step 1: Understand PHP array key behavior

   When an array has duplicate keys, the last value overwrites previous ones. Here, 'age' appears twice with values 25 and 30, so 30 is kept.

2. Step 2: Apply extract() and echo

   Extract creates variables from keys, so $age will be 30, the last value. Echoing $age prints 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Duplicate keys keep last value; extract uses that [OK]

Hint: Last duplicate key value is used in arrays [OK]

Common Mistakes:

• Expecting first duplicate key value to be used
• Thinking extract throws error on duplicates
• Assuming undefined variable error due to duplicates