Palindrome Number Triangle in PHP

What You’ll Learn

The palindrome number triangle prints 1, 121, 12321, 1234321, 123454321 — a natural step after the diagonal asterisk pattern in Program 26. This tutorial covers ascending and descending inner loops, mirroring, a live preview, algorithm steps, worked PHP examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer rows (e.g. 5), print a palindrome triangle: for each i, print 1..i then i-1..1 on the same line.

// rows = 5 (conceptual shape)
// 1
// 121
// 12321
// 1234321
// 123454321

Inputs & Outputs

Minimal workflow

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

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the palindrome number triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete PHP programs — fixed rows, user input, and spaced output variant. Click View Output to reveal sample console results.

<?php
for ($i = 1; $i <= 5; $i++) {
    for ($j = 1; $j <= $i; $j++) {
        echo $j;
    }

    for ($k = $i - 1; $k >= 1; $k--) {
        echo $k;
    }

    echo PHP_EOL;
}

1
121
12321
1234321
123454321

<?php
echo "Enter rows: ";
$rows = (int) trim(fgets(STDIN));

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

    for ($k = $i - 1; $k >= 1; $k--) {
        echo $k;
    }

    echo PHP_EOL;
}

Enter rows: 4
1
121
12321
1234321

<?php
$rows = 5;

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

    for ($k = $i - 1; $k >= 1; $k--) {
        echo $k . " ";
    }

    echo PHP_EOL;
}

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

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, the ascending and descending halves, and the full row output.

Row length grows as 2i - 1 digits — total prints = n² for n rows.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices