For Loop

What Is a for Loop?

In simple terms: A for loop repeats code a specific number of times. You set where a counter starts, when it stops, and how it changes — all in one line.

Technically: The for loop has a header with three expressions: an initialization that runs once, a condition checked before every iteration, and an update that runs after every iteration. It is the most compact way to write a counting loop.

Why It Is Needed

When the number of repetitions is known — or depends on a numeric range — a for loop keeps all the counter logic in one place:

for ($i = 1; $i <= 5; $i++) { ... }

Compare this to a while loop, where the same three parts are spread across separate lines. With for, a reader immediately sees the start, the end, and the step.

Syntax

for (initialization; condition; update) {
    // code to repeat
}

Flow

initialization
      │
      ▼
 ┌─────────────┐  false
 │ condition ? │────────► continue after the loop
 └─────┬───────┘
     true
       ▼
   loop body
       │
       ▼
    update ────► back to condition

Example

<?php
for ($i = 1; $i <= 5; $i++) {
    echo "Row $i\n";
}

Output

Row 1
Row 2
Row 3
Row 4
Row 5

How It Works

1. $i = 1 runs once.
2. $i <= 5 is checked: true, so the body runs.
3. After the body, $i++ increases $i to 2, and the condition is checked again.
4. When $i becomes 6, the condition is false and the loop ends.

After the loop, $i still exists and holds 6. PHP has no block scope, so loop variables remain available after the loop.

Counting Patterns

Counting Down

<?php
for ($i = 5; $i >= 1; $i--) {
    echo $i . ' ';
}
echo "\n";

Output

5 4 3 2 1 

Steps Other Than 1

<?php
for ($percent = 0; $percent <= 100; $percent += 25) {
    echo "$percent% ";
}
echo "\n";

for ($i = 10; $i >= 0; $i -= 2) {
    echo $i . ' ';
}
echo "\n";

Output

0% 25% 50% 75% 100% 
10 8 6 4 2 0 

The update expression can be any assignment, such as += 25 or -= 2.

Multiple Expressions

Each part of the header can contain several expressions separated by commas:

<?php
for ($left = 0, $right = 10; $left < $right; $left += 3, $right -= 3) {
    echo "left = $left, right = $right\n";
}

Output

left = 0, right = 10
left = 3, right = 7

Two counters move toward each other. After the second iteration, $left becomes 6 and $right becomes 4, so $left < $right is false. (When the condition part contains several comma-separated expressions, only the last one decides whether the loop continues — use && instead to avoid confusion.)

Looping Over Indexed Arrays

A for loop can walk through an array with numeric keys 0, 1, 2, …:

<?php
$fruits = ['Mango', 'Banana', 'Guava'];

for ($i = 0, $n = count($fruits); $i < $n; $i++) {
    echo ($i + 1) . ". {$fruits[$i]}\n";
}

Output

1. Mango
2. Banana
3. Guava

Why It Is Written This Way

• Array indexes start at 0, so the loop starts at $i = 0 and uses < (not <=) with the count.
• $n = count($fruits) is calculated once in the initialization rather than on every iteration in the condition.
• $i + 1 produces human-friendly numbering starting at 1.

For simply visiting every element, foreach is clearer and works with any keys. for is useful when you need the index for calculations — such as stepping through every second element, comparing neighbors, or iterating backwards.

Iterating Backwards

<?php
$history = ['Page A', 'Page B', 'Page C'];

for ($i = count($history) - 1; $i >= 0; $i--) {
    echo "Back to: {$history[$i]}\n";
}

Output

Back to: Page C
Back to: Page B
Back to: Page A

The last index is count - 1, because indexes start at 0.

Empty Header Parts

Any of the three parts can be left empty. An empty condition is treated as true, so for (;;) is an infinite loop — equivalent to while (true) — and needs a break:

<?php
$attempt = 0;

for (;;) {
    $attempt++;

    if ($attempt === 3) {
        echo "Stopped after $attempt attempts\n";
        break;
    }
}

Output

Stopped after 3 attempts

while (true) expresses the same idea more readably and is more common.

Practical Example: Next 7 Days

<?php
$start = new DateTimeImmutable('2026-10-09');

for ($day = 0; $day < 7; $day++) {
    $date = $start->modify("+$day day");
    $label = $day === 0 ? ' (today)' : '';

    echo $date->format('D d M') . $label . "\n";
}

Output

Fri 09 Oct (today)
Sat 10 Oct
Sun 11 Oct
Mon 12 Oct
Tue 13 Oct
Wed 14 Oct
Thu 15 Oct

How It Works

• The counter $day runs from 0 to 6 — exactly seven iterations.
• modify("+$day day") returns a new DateTimeImmutable that is $day days after the start. The original $start is never changed.
• format('D d M') produces a short weekday, the day of the month, and a short month name.

This pattern is used for date pickers, delivery slot selection, and calendars.

Practical Example: Quantity Price Table

<?php
$unitPrice = 250;

printf("%-5s %-10s %s\n", 'Qty', 'Discount', 'Total');

for ($qty = 1; $qty <= 5; $qty++) {
    $discountPercent = $qty >= 5 ? 15 : ($qty >= 3 ? 10 : 0);
    $total = $qty * $unitPrice * (100 - $discountPercent) / 100;

    printf("%-5d %-10s ₹%s\n", $qty, $discountPercent . '%', number_format($total, 2));
}

Output

Qty   Discount   Total
1     0%         ₹250.00
2     0%         ₹500.00
3     10%        ₹675.00
4     10%        ₹900.00
5     15%        ₹1,062.50

Each iteration calculates one row of the table for a quantity from 1 to 5. The nested ternary is parenthesized, as PHP 8 requires.

Common Mistakes

Off-by-One With Array Indexes

<?php
$items = ['A', 'B', 'C'];

for ($i = 0; $i <= count($items); $i++) {
    echo $items[$i];
}

Output

ABC
PHP Warning:  Undefined array key 3 in /path/script.php on line 5

<= runs one iteration too many. Use < with the count, or use foreach.

Using Floats as Loop Counters

for ($x = 0; $x != 1; $x += 0.1) { ... }

Effect: 0.1 cannot be represented exactly, so $x never equals 1 exactly — this loop never ends.

Fix: Count with integers and calculate the float inside the loop: for ($i = 0; $i <= 10; $i++) { $x = $i / 10; ... }.

Changing the Counter Inside the Body

Effect: Skipped or repeated iterations that are hard to follow.

Fix: Change the counter only in the update part of the header.

Calling Expensive Functions in the Condition

Effect: The function runs on every iteration.

Fix: Calculate the value once in the initialization ($i = 0, $n = count($items)).

Using for With Associative Arrays

Effect: Numeric keys do not exist, causing warnings.

Fix: Use foreach.

Best Practices

• Use for when the number of iterations is known or follows a numeric range.
• Use foreach to iterate over arrays unless you need index arithmetic.
• Count with integers; compute floats from the counter.
• Use < with zero-based counts to avoid off-by-one errors.
• Keep the header simple — one counter, one condition, one update — whenever possible.

Official reference: https://www.php.net/manual/en/control-structures.for.php