PHP Palindrome Number Triangle Pattern

What Is This Pattern?

A palindrome number triangle prints ascending digits 1..$i, then mirrors with $i-1..1 — each row reads the same forwards and backwards.

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

1
121
12321
1234321
123454321     ← rows = 5

Follows the diagonal asterisk pattern in Program 26; next is the 0-centered descending mirror in Program 28.

How to Solve It

Outer loop grows $i. First inner loop prints 1..$i; second prints $i-1..1 so the peak digit appears once.

Pseudocode

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

Cheat sheet

Printing Numbers vs Starting a New Line

Print digits without a newline, then end the row once.

Live Preview

Change the row count and the palindrome triangle updates instantly — capped at 9 for readable demos.

1
121
12321
1234321
123454321

Worked Walkthrough — $rows = 5

Trace how ascending then descending halves build each palindrome row.

Row $i prints 2i - 1 digits. Total = 1 + 3 + … + (2n-1) = n² → O(n²).

PHP Programs

Three complete programs: fixed $rows = 5, CLI input, and a compact $rows = 3 demo. Use View Output to reveal sample results.

<?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
121
12321
1234321
123454321

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

if (!is_numeric($input) || (int) $input <= 0) {
    echo "Please enter a positive integer." . PHP_EOL;
    exit(1);
}

$rows = (int) $input;

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

if (!is_numeric($input) || (int) $input < 1 || (int) $input > 9) {
    echo "Enter a whole number from 1 to 9." . PHP_EOL;
    exit(1);
}

<?php
$rows = 3;

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
121
12321

Time and Space Complexity

Row $i prints 2i - 1 digits. Total = 1 + 3 + … + (2n-1) = n² → O(n²) time. Only a few loop variables are needed.

Key Takeaways