Diagonal Mirror Number Pattern in PHP

What You’ll Learn

The diagonal mirror pattern prints numbers on the main diagonal and mirror diagonal with spaces elsewhere — forming an X shape. For $n = 5: 1 1, 2 2, 3 3. This tutorial covers $i==$j and $i==$k logic, live preview, worked PHP examples, edge cases, and O(n²) complexity.

📝 Problem & Approach

Given $n = 5, print five lines with digits on both diagonals and spaces elsewhere.

// $n = 5 (conceptual output)
// 1       1
//  2     2 
//   3   3  
//    4 4   
//     5    

Inputs & Outputs

Minimal workflow

for i from 1 to n:
    for j from 1 to n: print j if $i==$j else space
    for k from n-1 down to 1: print k if $i==$k else space
    newline

Approach comparison

🔮 Live Preview

Choose pattern size $n and draw the diagonal mirror number pattern in the browser.

Press "Draw pattern".

Examples Gallery

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

<?php
$n = 5;

for ($i = 1; $i <= $n; $i++) {
    for ($j = 1; $j <= $n; $j++) {
        if ($i == $j) echo $j;
        else echo " ";
    }
    for ($k = $n - 1; $k >= 1; $k--) {
        if ($i == $k) echo $k;
        else echo " ";
    }
    echo PHP_EOL;
}

1       1
 2     2 
  3   3  
   4 4   
    5    

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

for ($i = 1; $i <= $n; $i++) {
    for ($j = 1; $j <= $n; $j++) {
        echo $i == $j ? $j : " ";
    }
    for ($k = $n - 1; $k >= 1; $k--) {
        echo $i == $k ? $k : " ";
    }
    echo PHP_EOL;
}

Enter n: 5
1       1
 2     2 
  3   3  
   4 4   
    5    

<?php
$n = 5;

for ($i = 1; $i <= $n; $i++) {
    for ($j = 1; $j <= $n; $j++) {
        echo $i == $j ? "*" : " ";
    }
    for ($k = $n - 1; $k >= 1; $k--) {
        echo $i == $k ? "*" : " ";
    }
    echo PHP_EOL;
}

*       *
 *     * 
  *   *  
   * *   
    *    

🔎 Worked Walkthrough — row $i = 3, $n = 5

Trace row 3 to see how main and mirror diagonal checks place digits at both X arms.

Final row: 3 3. Then 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