Why Is This Important?

Rounding and generating random numbers are the foundation of many tasks: calculators, shopping carts, pagination, games, A/B testing.
round, ceil, and floor help control calculation precision and how numbers are displayed.
rand (and modern alternatives) are used to generate random numbers in PHP.

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Basics: What Do These Functions Do?

round(float \$num, int \$precision = 0, int \$mode = PHPROUNDHALFUP): float

Rounds a number to the given decimal places (precision) and using a specified rounding mode. By default: “half up” rounding.

ceil(float \$num): float

Returns the smallest integer greater than or equal to the given number (rounds up).

floor(float \$num): float

Returns the largest integer less than or equal to the given number (rounds down).

rand(int \$min, int \$max): int

Returns a random integer from the range \[min, max] (both inclusive).
⚠️ Note: ceil and floor always round to integers. round lets you control decimal precision and how halves (e.g., 1.5) are treated.

PHP Code Examples

round — rounding to precision and mode

<?php // Basic rounding to nearest integer echo round(3.2); // 3 echo PHP_EOL; echo round(3.5); // 4 (default half up) echo PHP_EOL; // Decimal places (precision): echo round(3.14159, 2); // 3.14 echo PHP_EOL; echo round(3.145, 2); // 3.15 (half up) echo PHP_EOL; // Negative precision — tens, hundreds, etc.: echo round(1234.567, -1); // 1230 echo PHP_EOL; echo round(1234.567, -2); // 1200 echo PHP_EOL; // Different rounding modes echo round(2.5, 0, PHP_ROUND_HALF_UP); // 3 echo PHP_EOL; echo round(2.5, 0, PHP_ROUND_HALF_DOWN); // 2 echo PHP_EOL; echo round(2.5, 0, PHP_ROUND_HALF_EVEN); // 2 (banker’s rounding) echo PHP_EOL; echo round(2.5, 0, PHP_ROUND_HALF_ODD); // 3 echo PHP_EOL; // round() always returns float var_dump(round(10.0)); // float(10)

⚠️ Floating point caveat: numbers like 2.675 can’t be represented exactly in binary:

<?php echo round(2.675, 2); // often 2.67 instead of 2.68 (due to float representation)

Solutions: work in cents (integers), use BCMath/Brick\Math, or format output (numberformat) instead of true rounding.

ceil — rounding up

<?php
echo ceil(3.01);  // 4
echo PHP_EOL;
echo ceil(3.99);  // 4
echo PHP_EOL;
echo ceil(3.0);   // 3
echo PHP_EOL;

// Negative numbers:
echo ceil(-3.1);  // -3 (towards zero)
echo PHP_EOL;
echo ceil(-3.9);  // -3

floor — rounding down

<?php
echo floor(3.99); // 3
echo PHP_EOL;
echo floor(3.01); // 3
echo PHP_EOL;

// Negative numbers:
echo floor(-3.1); // -4 (away from zero)
echo PHP_EOL;
echo floor(-3.9); // -4

floor/ceil vs casting to int

<?php
echo (int) 3.9;   // 3 (truncates towards zero)
echo PHP_EOL;
echo (int) -3.9;  // -3 (differs from floor(-3.9) == -4)

Trick: ceil/floor to decimal places

Since ceil and floor don’t have precision parameter, scale the number:

<?php
$val = 12.345;

echo ceil($val * 100) / 100;  // 12.35
echo PHP_EOL;
echo floor($val * 100) / 100; // 12.34

numberformat vs round — formatting output

For nice display of prices/numbers, prefer formatting:

<?php $price = 1234.5; echo number_format($price, 2, ',', ' '); // "1 234,50"

numberformat doesn’t change the numeric value, only returns a string for display.

rand — random numbers

<?php
// Dice roll: 1–6
echo rand(1, 6);
echo PHP_EOL;

// Random index
$colors = ['red', 'green', 'blue'];
$randomIndex = rand(0, count($colors) - 1);
echo $colors[$randomIndex];
echo PHP_EOL;

// Random float [0,1)
$float = rand(0, PHP_INT_MAX) / PHP_INT_MAX;
echo $float;

Modern PHP (7.1+) implements rand using Mersenne Twister, but it’s not cryptographically secure.

For security (tokens, passwords, keys), use:

<?php
$safe = random_int(1, 100);
$bytes = random_bytes(16);
$token = bin2hex($bytes);
echo $token;

Best Practices and Common Mistakes

Best Practices

For finance, avoid floats: use integers (cents) or BCMath/Brick\Math.
For display, prefer numberformat over round.

Pick rounding mode intentionally (e.g., banker’s rounding in accounting).

Be careful with negatives in ceil/floor (up/down are relative to number line).

For secure randomness: use randomint and randombytes, not rand.

Remember rand’s inclusive range \[min, max].

Common Mistakes

Expecting round(2.675, 2) to give 2.68 — float limitation.

Confusing casting (int)\$x with floor/ceil for negatives.

Using rand for cryptography (insecure).

Rounding multiple times unnecessarily instead of once at output.

Assuming ceil/floor/round return int — in PHP, they return float. Cast explicitly if needed.

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Summary

round: flexible rounding with precision and mode (HALFUP, HALFDOWN, HALFEVEN, HALFODD).

ceil: up to next integer; for negatives, towards zero.

floor: down to previous integer; for negatives, away from zero.

rand: simple random ints (not secure). Use randomint/randombytes for secure randomness.

Use numberformat for display, and int/BCMath for financial calculations.

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Mini Quiz

What does round(2.5) return? → 3
What does round(2.5, 0, PHPROUNDHALFEVEN) return? → 2

What does ceil(-3.2) return? → -3

What does floor(-3.2) return? → -4

Difference: (int)-4.9 vs floor(-4.9)? → -3 vs -5

round(1234.567, -2)? → 1200

Can rand(1,10) return 10? → Yes

Secure random integer? → randomint
Cut to 2 decimals without rounding up? → floor(\$x \* 100)/100
Best way to display formatted price? → number_format(\$price, 2, ',', ' ')

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Now you know how to use the essential math functions in PHP safely and effectively.