Compact and extract functions in PHP - Step-by-Step Execution

Concept Flow - Compact and extract functions

Define variables

↓

Call compact()

↓

Create array with variable names as keys

↓

Call extract()

↓

Create variables from array keys

↓

Use variables

↓

End

Variables are packed into an array with compact(), then unpacked back into variables with extract().

Execution Sample

PHP

<?php
$a = 10;
$b = 20;
$arr = compact('a', 'b');
extract($arr);
echo $a + $b;
?>

This code packs variables a and b into an array, then extracts them back, and prints their sum.

Execution Table

Step	Action	Variables Before	Result	Variables After	Output
1	Define $a=10, $b=20	none	Variables created	$a=10, $b=20
2	Call compact('a','b')	$a=10, $b=20	Array ['a'=>10, 'b'=>20]	$a=10, $b=20, $arr=['a'=>10,'b'=>20]
3	Call extract($arr)	$a=10, $b=20, $arr=['a'=>10,'b'=>20]	Variables $a and $b set from array	$a=10, $b=20, $arr=['a'=>10,'b'=>20]
4	Calculate echo $a + $b	$a=10, $b=20, $arr=['a'=>10,'b'=>20]	Sum is 30	$a=10, $b=20, $arr=['a'=>10,'b'=>20]	30
5	End of script	$a=10, $b=20, $arr=['a'=>10,'b'=>20]	Script ends	$a=10, $b=20, $arr=['a'=>10,'b'=>20]

💡 Script ends after printing the sum 30.

Variable Tracker

Variable	Start	After compact	After extract	Final
$a	undefined	10	10	10
$b	undefined	20	20	20
$arr	undefined	['a'=>10,'b'=>20]	['a'=>10,'b'=>20]	['a'=>10,'b'=>20]

Key Moments - 3 Insights

Why does extract() create variables even if they already exist?

extract() overwrites existing variables with values from the array, as shown in step 3 of the execution_table.

What happens if compact() is called with a variable name that does not exist?

compact() ignores non-existing variables and does not include them in the array, so they won't be extracted later.

Does extract() create variables in the current scope or globally?

extract() creates variables in the current scope, so they can be used immediately after extraction, as seen in step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what does compact('a','b') return?

A An array with keys 'a' and 'b' and their values

B A string concatenation of $a and $b

C An error because variables are not passed by reference

D An empty array

💡 Hint

Check the Result column at step 2 in execution_table.

At which step does extract() set variables from the array?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the extract() call in the Action column in execution_table.

If $b was not defined before compact(), what would $arr contain?

A Empty array

B Array with 'a' and 'b' keys, 'b' value null

C Array with only 'a' key and its value

D Array with 'b' key only

💡 Hint

Recall compact() ignores variables that do not exist, see key_moments explanation.

Concept Snapshot

compact('var1', 'var2', ...): creates an array with variable names as keys and their values.
extract(array): creates variables from array keys in current scope.
Use compact to pack variables, extract to unpack.
extract overwrites existing variables.
Useful for passing variables around easily.

Full Transcript

This example shows how PHP's compact() function takes variable names as strings and creates an array with those variables' values. Then extract() takes that array and creates variables in the current scope with the array's keys as variable names. The code defines $a=10 and $b=20, packs them into $arr with compact, then extracts them back. Finally, it prints the sum 30. The execution table traces each step, showing variable states and outputs. Key moments clarify common confusions about variable scope and behavior of these functions.

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