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.