PHP Hollow Square Pattern (Border of 1s)

What Is This Pattern?

A hollow square border prints 1 only on the edges of an n × n grid and leaves the interior as spaces.

Rule: print 1 on the border; spaces inside

1 1 1 1 1
1       1
1       1
1       1
1 1 1 1 1   ← size = 5

In PHP loop $i and $j from 1 to $size. Print "1 " when the cell is on the border; otherwise print " ".

How to Solve It

One nested loop over every cell, plus a four-way border check.

Pseudocode

for i from 1 to size:
    for j from 1 to size:
        if i is 1 or size, or j is 1 or size:
            print "1 "
        else:
            print "  "
    print newline

Cheat sheet

Printing Numbers vs Starting a New Line

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

Live Preview

Change the square size and the hollow border updates instantly.

1 1 1 1 1 
1       1 
1       1 
1       1 
1 1 1 1 1 

Worked Walkthrough — $size = 4

Trace each row: top and bottom are solid; middle rows keep only the ends.

Border cells for size n = 4(n - 1) when n >= 2 (corners counted once).

PHP Programs

Three complete programs: fixed 5×5, CLI input size, and an asterisk-border variant. Use View Output for sample results.

<?php
$size = 5;

for ($i = 1; $i <= $size; $i++) {
    for ($j = 1; $j <= $size; $j++) {
        if ($i == 1 || $i == $size || $j == 1 || $j == $size)
            echo "1 ";
        else
            echo "  ";
    }

    echo PHP_EOL;
}

1 1 1 1 1 
1       1 
1       1 
1       1 
1 1 1 1 1 

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

if (!is_numeric($input) || (int) $input < 2) {
    echo "Please enter an integer greater than 1." . PHP_EOL;
    exit(1);
}

$size = (int) $input;

for ($i = 1; $i <= $size; $i++) {
    for ($j = 1; $j <= $size; $j++) {
        if ($i == 1 || $i == $size || $j == 1 || $j == $size)
            echo "1 ";
        else
            echo "  ";
    }

    echo PHP_EOL;
}

Enter square size: 4
1 1 1 1 
1     1 
1     1 
1 1 1 1 

<?php
$size = 5;

for ($i = 1; $i <= $size; $i++) {
    for ($j = 1; $j <= $size; $j++) {
        if ($i == 1 || $i == $size || $j == 1 || $j == $size)
            echo "* ";
        else
            echo "  ";
    }

    echo PHP_EOL;
}

* * * * * 
*       * 
*       * 
*       * 
* * * * * 

Time and Space Complexity

Every cell of the n × n grid is visited once, so work is quadratic in the side length.

Key Takeaways