Array access and modification in PHP - Step-by-Step Execution

Concept Flow - Array access and modification

Start

↓

Access array element by index/key

↓

Read or modify value

↓

Store back if modified

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Continue or end

This flow shows how to access an element in an array by its index or key, then read or change its value, and store it back if modified.

Execution Sample

PHP

<?php
$array = [10, 20, 30];
$array[1] = 25;
echo $array[1];
?>

This code changes the second element of the array from 20 to 25 and then prints it.

Execution Table

Step	Action	Array State	Accessed Index	Value Read	Value Written	Output
1	Initialize array	[10, 20, 30]	-	-	-	-
2	Access index 1	[10, 20, 30]	1	20	-	-
3	Modify index 1 to 25	[10, 25, 30]	1	20	25	-
4	Access index 1 to print	[10, 25, 30]	1	25	-	25
5	End	[10, 25, 30]	-	-	-	-

💡 Finished all steps, array modified and output printed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
$array	[10, 20, 30]	[10, 20, 30]	[10, 25, 30]	[10, 25, 30]	[10, 25, 30]
Accessed Value	-	20	20	25	-
Output	-	-	-	25	25

Key Moments - 3 Insights

Why does changing $array[1] = 25; affect the original array?

Because arrays in PHP are mutable, assigning a new value to $array[1] directly updates the element at index 1, as shown in step 3 of the execution table.

What happens if we try to access an index that does not exist?

PHP will give a notice about undefined offset and return null or empty value. This is not shown here but is important to avoid by checking if the index exists before accessing.

Why do we print $array[1] after modification?

To confirm the modification took effect. Step 4 accesses the updated value 25 and outputs it, showing the change.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $array after step 3?

A [25, 20, 30]

B [10, 20, 30]

C [10, 25, 30]

D [10, 30, 25]

💡 Hint

Check the 'Array State' column at step 3 in the execution table.

At which step is the value 20 read from the array?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Value Read' column in the execution table.

If we changed $array[1] = 30; at step 3, what would be the output at step 4?

A 25

B 30

C 20

D 10

💡 Hint

The output matches the value written at step 3, check 'Value Written' and 'Output' columns.

Concept Snapshot

Array access and modification in PHP:
- Use $array[index] to access elements.
- Assign new value with $array[index] = value;
- Accessing non-existing index gives a notice.
- Changes affect the original array immediately.
- Use echo to print array elements.

Full Transcript

This visual trace shows how PHP arrays are accessed and modified. We start with an array of three numbers. We read the value at index 1, which is 20. Then we change it to 25. Finally, we print the updated value 25. The variable tracker shows how the array changes after each step. Key moments clarify why the array changes and what happens if we access invalid indexes. The quiz tests understanding of array state and values at each step.