Growing Reverse Number Triangle in PHP

What You’ll Learn

The Growing Reverse Number Triangle prints 1, 21, 321, 4321, 54321 — row $i concatenates digits $i down to 1. This tutorial covers nested-loop logic, live preview, worked PHP examples, edge cases, and O(n²) complexity.

📝 Problem & Approach

Given a positive integer $rows (e.g. 5), print a growing reverse Number Triangle: row $i prints digits $i down to 1 with no spaces (e.g. row 3 → 321).

// $rows = 5 (conceptual shape)
for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j >= 1; $j--) {
        echo $j;          // digits i..1
    }
    echo PHP_EOL;         // next row
}

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    for j from i down to 1:
        print j
    print newline

Approach comparison

🔮 Live Preview

Enter a row count and draw the Growing Reverse Number Triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete PHP programs — fixed $rows = 5, fgets(STDIN) input, and a spaced-output variant. Click View Output to reveal sample console results.

<?php
$rows = 5;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j >= 1; $j--) {
        echo $j;
    }
    echo PHP_EOL;
}

1
21
321
4321
54321

<?php
echo "Enter the number of rows: ";
$input = trim(fgets(STDIN));
if (!is_numeric($input)) {
    echo "Invalid input." . PHP_EOL;
    exit(1);
}
$rows = (int) $input;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j >= 1; $j--) {
        echo $j;
    }
    echo PHP_EOL;
}

Enter the number of rows: 4
1
21
321
4321

<?php
$rows = 5;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j >= 1; $j--) {
        echo $j . " ";
    }
    echo PHP_EOL;
}

1
2 1
3 2 1
4 3 2 1
5 4 3 2 1

🔎 Worked Walkthrough — row $i = 3

Trace row 3 to see how the inner loop builds 321.

After the inner loop, echo PHP_EOL moves to the next row.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices