Array push and pop in PHP - Step-by-Step Execution

Concept Flow - Array push and pop

Start with empty array

↓

Push element

↓

Array grows by 1

↓

Push more elements?

Yes→Repeat Push

No↓

Pop element

↓

Array shrinks by 1

↓

Pop more elements?

Yes→Repeat Pop

No↓

End

Start with an array, add elements with push to grow it, then remove elements with pop to shrink it.

Execution Sample

PHP

<?php
$array = [];
array_push($array, 10);
array_push($array, 20);
$last = array_pop($array);
print_r($array);
?>

This code adds 10 and 20 to an array, removes the last element, then prints the array.

Execution Table

Step	Action	Array State	Returned Value	Notes
1	Initialize empty array	[]	N/A	Array starts empty
2	Push 10	[10]	N/A	10 added to array
3	Push 20	[10, 20]	N/A	20 added to array
4	Pop last element	[10]	20	20 removed and returned
5	Print array	[10]	N/A	Array contains one element 10

💡 No more push or pop operations, execution ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
$array	[]	[10]	[10, 20]	[10]	[10]
$last	undefined	undefined	undefined	20	20

Key Moments - 3 Insights

Why does array_pop return the last element but also remove it from the array?

array_pop both returns and removes the last element, as shown in step 4 of execution_table where the array shrinks and the popped value is stored in $last.

What happens if we pop from an empty array?

Popping from an empty array returns NULL and the array stays empty. This is not shown here but is important to know.

Why does array_push add elements to the end of the array?

array_push always adds elements at the end, growing the array as seen in steps 2 and 3 where elements 10 and 20 are added at the end.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the value of $last after popping?

A 10

B 20

C NULL

D Array

💡 Hint

Check the 'Returned Value' column at step 4 in execution_table

At which step does the array first contain two elements?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Array State' column in execution_table for each step

If we pop again after step 4, what will be the array state?

A [20]

B [10]

C []

D ['10', '20']

💡 Hint

After popping 20, popping again removes 10, leaving an empty array

Concept Snapshot

array_push($array, value) adds value to the end of array
array_pop($array) removes and returns the last element
Push grows the array; pop shrinks it
Popping empty array returns NULL
Use these to treat arrays like stacks

Full Transcript

We start with an empty array. Using array_push, we add 10, then 20, so the array grows to [10, 20]. Then array_pop removes the last element 20 and returns it, leaving the array as [10]. Finally, printing the array shows it contains one element 10. This shows how push adds to the end and pop removes from the end, like a stack.

Practice

1. What does the PHP function array_push do to an array?

easy

A. Adds one or more elements to the end of the array

B. Removes the first element from the array

C. Removes the last element from the array

D. Sorts the array in ascending order

Solution

1. Step 1: Understand the purpose of array_push

   array_push is used to add elements to the end of an existing array.

2. Step 2: Compare with other array functions

   Functions like array_pop remove elements, but array_push only adds elements at the end.

3. Final Answer:

   Adds one or more elements to the end of the array -> Option A

4. Quick Check:

   array_push = add elements at end [OK]

Hint: Remember: push means add at the end [OK]

Common Mistakes:

• Confusing array_push with array_pop
• Thinking array_push removes elements
• Assuming array_push sorts the array

2. Which of the following is the correct syntax to add an element 'apple' to the array $fruits using array_push?

easy

A. array_push('apple', $fruits);

B. array_push($fruits['apple']);

C. array_push($fruits => 'apple');

D. array_push($fruits, 'apple');

Solution

1. Step 1: Recall the function signature

   The correct syntax is array_push(array, value1, value2, ...), where the first argument is the array variable.

2. Step 2: Check each option

   array_push($fruits, 'apple'); correctly uses array_push($fruits, 'apple');. Others misuse argument order or syntax.

3. Final Answer:

   array_push($fruits, 'apple'); -> Option D

4. Quick Check:

   array_push(array, value) syntax [OK]

Hint: First argument is array, then values to add [OK]

Common Mistakes:

• Swapping argument order
• Using arrow => instead of comma
• Passing only one argument incorrectly

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

$colors = ['red', 'green'];
array_push($colors, 'blue');
$last = array_pop($colors);
print_r($colors);
echo $last;

medium

A. Array ( [0] => red [1] => green [2] => blue ) blue

B. Array ( [0] => red [1] => green ) blue

C. Array ( [0] => red [1] => green ) green

D. Array ( [0] => red [1] => green [2] => blue ) green

Solution

1. Step 1: Understand array_push effect

   array_push adds 'blue' to the end, so array becomes ['red', 'green', 'blue'].

2. Step 2: Understand array_pop effect

   array_pop removes and returns the last element 'blue', so array becomes ['red', 'green'] and $last = 'blue'.

3. Final Answer:

   Array ( [0] => red [1] => green ) blue -> Option B

4. Quick Check:

   push adds, pop removes last [OK]

Hint: Push adds last, pop removes last [OK]

Common Mistakes:

• Thinking pop removes first element
• Expecting array to still have 'blue'
• Confusing printed array and echoed value

4. Identify the error in the following PHP code snippet:

$numbers = [1, 2, 3];
array_pop($numbers, 2);
print_r($numbers);

medium

A. No error, code works fine

B. array_pop should be called before array initialization

C. array_pop accepts only one argument, extra argument causes error

D. array_pop cannot be used on numeric arrays

Solution

1. Step 1: Check array_pop function signature

   array_pop accepts only one argument: the array variable.

2. Step 2: Analyze the code

   Passing a second argument (2) is invalid and will cause an error.

3. Final Answer:

   array_pop accepts only one argument, extra argument causes error -> Option C

4. Quick Check:

   array_pop(array) only one argument [OK]

Hint: array_pop takes exactly one argument [OK]

Common Mistakes:

• Passing multiple arguments to array_pop
• Thinking array_pop removes multiple elements
• Assuming no error on extra arguments

5. Given an empty array $stack = [];, which sequence of PHP statements correctly adds 'first', 'second', then removes the last added element and stores it in $removed?

hard

A. array_push($stack, 'first'); array_push($stack, 'second'); $removed = array_pop($stack);

B. $removed = array_pop($stack); array_push($stack, 'first'); array_push($stack, 'second');

C. array_pop($stack, 'first'); array_pop($stack, 'second'); $removed = array_push($stack);

D. array_push('first', $stack); array_push('second', $stack); $removed = array_pop($stack);

Solution

1. Step 1: Add elements in correct order

   Use array_push($stack, 'first') and then array_push($stack, 'second') to add elements to the stack.

2. Step 2: Remove last element and store it

   Use $removed = array_pop($stack); to remove and save the last added element ('second').

3. Final Answer:

   array_push($stack, 'first'); array_push($stack, 'second'); $removed = array_pop($stack); -> Option A

4. Quick Check:

   Push adds, pop removes last [OK]

Hint: Push then pop to simulate stack behavior [OK]

Common Mistakes:

• Calling array_pop before pushing elements
• Passing arguments in wrong order
• Using array_pop with extra arguments