In PHP, how do sprintf() conversion specifications like %05d, %'*10s, %.2f and %1$s work when formatting invoice lines?

middleshould knowfreq 45%

answer

percent, argnum, flags, width, precision
0 flag pads numbers with zeros
apostrophe sets a custom pad character
%d truncates a float
too few values: ArgumentCountError

basics

~10 s

Each sprintf() specification is %[argnum$][flags][width][.precision]specifier: %05d zero-pads an integer to five characters, %'*10s pads a string with asterisks, %.2f prints two decimals and %1$s reuses the first argument. Too few values throw ArgumentCountError.

solid answer

~40 s

sprintf(string $format, mixed ...$values): string copies the format and replaces each conversion specification, shaped %[argnum$][flags][width][.precision]specifier. The specifier picks the conversion: d integer, s string, f or F float, x hex, % a literal percent. Width is a minimum length for the whole result, padded with spaces by default; the 0 flag pads numbers with zeros, ' followed by a character pads with that character, and - left-justifies. Precision means digits after the point for f, but a maximum length for s. argnum$ selects an argument by position, so %1$s can be reused or reordered. So sprintf('INV-%d-%05d', 2026, 42) gives INV-2026-00042. Watch for %d truncating 19.99 to 19, and since PHP 8.0 too few values throw ArgumentCountError instead of returning false.

`sprintf(string $format, mixed ...$values): string` copies the format and replaces each conversion specification, shaped `%[argnum$][flags][width][.precision]specifier`. The **specifier** picks the conversion: `d` integer, `s` string, `f` or `F` float, `x` hex, `%` a literal percent. **Width** is a minimum length for the whole result, padded with spaces by default; the `0` flag pads numbers with zeros, `'` followed by a character pads with that character, and `-` left-justifies. **Precision** means digits after the point for `f`, but a maximum length for `s`. `argnum$` selects an argument by position, so `%1$s` can be reused or reordered. So `sprintf('INV-%d-%05d', 2026, 42)` gives `INV-2026-00042`. Watch for `%d` truncating `19.99` to `19`, and since PHP 8.0 too few values throw `ArgumentCountError` instead of returning `false`.

code

php · 19 lines

php

<?php
declare(strict_types=1);

$year = 2026;
$seq = 42;
$net = 119.9;

echo sprintf('INV-%d-%05d', $year, $seq), "\n";   // INV-2026-00042
echo sprintf('%-10s|%10.2F|', 'Net', $net), "\n"; // Net       |    119.90|
echo sprintf("%'*12s", 'TOTAL'), "\n";            // *******TOTAL
echo sprintf('%.3s', 'INVOICE'), "\n";            // INV
echo sprintf('%d', 19.99), "\n";                  // 19 (truncated, not rounded)
echo sprintf('%2$s %1$s', 'EUR', '119.90'), "\n"; // 119.90 EUR

try {
    $line = sprintf('%s-%s', 'INV');
} catch (ArgumentCountError $e) {
    echo get_class($e), "\n";                      // ArgumentCountError
}

go deeper

for a junior

Recall the everyday specifiers d, s, f and %%, and how to zero-pad a number to a fixed width.

for a middle

Explain the full argnum, flags, width, precision, specifier grammar, why width counts the whole value, and the PHP 8 ArgumentCountError for missing values.

for a senior

Show you would keep money out of %d, use F rather than f for machine-readable exports, and give translators positional argnum formats.

for a principal

Decide where formatting belongs in a system that serves several locales and file formats, and which output paths must be locale-independent by contract.

The shape of a conversion specification

sprintf(string $format, mixed ...$values): string returns a new string built from $format. Ordinary characters are copied as they are; each conversion specification fetches one value and formats it. The manual gives the prototype:

%[argnum$][flags][width][.precision]specifier

Only the % and the specifier are required. The parts, in order:

argnum$: an integer followed by $ that selects which value to use, counting from 1.
flags: - left-justifies; + prints a plus sign on positive numbers; a space pads with spaces (the default); 0 pads numbers with zeros on the left; ' followed by any character pads with that character.
width: the minimum number of characters for the whole converted value, including sign and decimal point. Since PHP 8.0 it can be *, taking the width from an extra argument.
.precision: for e, E, f, F, the number of digits after the point (6 when omitted); for s, a cutoff, the maximum number of characters kept.
specifier: the conversion itself.

The specifiers you use every day

Specifier	Treats the value as	Example	Result
`d`	signed integer	`sprintf('%05d', 42)`	`00042`
`u`	unsigned integer	`sprintf('%u', 42)`	`42`
`s`	string	`sprintf('%-6s	', 'Net')`
`f`	float, locale-aware point	`sprintf('%.2f', 119.9)`	`119.90` in the C locale
`F`	float, always a `.`	`sprintf('%8.2F', 119.9)`	`119.90`
`x` / `X`	integer in hex	`sprintf('%04X', 255)`	`00FF`
`b`	integer in binary	`sprintf('%b', 5)`	`101`
`e`	scientific notation	`sprintf('%.1e', 1200)`	`1.2e+3`
`%`	a literal percent sign	`sprintf('%d%%', 20)`	`20%`

The difference between f and F matters for machine-readable output: f uses the decimal point of the current numeric locale, F never does.

Formatting an invoice line

An accounting export typically needs fixed-width, zero-padded identifiers and amounts with a fixed number of decimals:

Invoice number: sprintf('INV-%d-%05d', $year, $sequence) turns 2026 and 42 into INV-2026-00042.
Amount column: sprintf('%10.2F', $net) right-aligns the amount in ten characters with two decimals.
Label column: sprintf('%-12s', $label) left-aligns a label and pads it with spaces on the right.
Filler: sprintf("%'.20s", $total) pads with dots, a common look for printed totals.
Reordering: sprintf('%2$s %1$s', 'EUR', '119.90') prints 119.90 EUR; translators can move arguments without changing the calling code.

Remember that width counts every character: sprintf('%06.2f', 3.14159) is 003.14, six characters including the point and both decimals, not six digits before the point.

Traps

%d truncates floats. The value is converted to an integer first, so sprintf('%d', 19.99) gives 19, not 20. Format amounts with f/F, or keep them as integer minor units.
Precision on s cuts. sprintf('%.3s', 'INVOICE') returns INV; it does not pad.
Too few values. Since PHP 8.0 a format that needs more values than you pass throws ArgumentCountError; before 8.0 it returned false with a warning. An argnum of zero throws ValueError.
A literal percent sign must be written %%; a lone % starts a specification.

The rest of the family

printf() writes the result to output and returns its length as an int.
vsprintf() and vprintf() take the values as one array, handy when they are already collected.
fprintf() writes to a stream resource, for example an open export file.

Since PHP 8.4, a sprintf() call that uses only %s and %d is compiled into the equivalent string interpolation, so simple formats cost no more than concatenation. For amounts shown to people, number_format() adds thousands separators, which sprintf() never does.

What interviewers listen for

A good answer reads the prototype aloud and applies it to a concrete line rather than listing specifiers from memory. Expect probing on three points:

Width versus precision: width is a minimum for the whole value, precision is decimals for floats and a cutoff for strings.
Integer conversion: %d on a money amount truncates, which is a silent data bug rather than a crash.
Failure mode: in PHP 8 a missing value is an ArgumentCountError, so a broken format fails loudly in tests instead of writing false into an export file.

Mentioning that F exists because f is locale-aware shows you have shipped files that other systems parse.

## The shape of a conversion specification `sprintf(string $format, mixed ...$values): string` returns a new string built from `$format`. Ordinary characters are copied as they are; each **conversion specification** fetches one value and formats it. The manual gives the prototype: ``` %[argnum$][flags][width][.precision]specifier ``` Only the `%` and the specifier are required. The parts, in order: - **argnum$**: an integer followed by `$` that selects which value to use, counting from 1. - **flags**: `-` left-justifies; `+` prints a plus sign on positive numbers; a space pads with spaces (the default); `0` pads numbers with zeros on the left; `'` followed by any character pads with that character. - **width**: the minimum number of characters for the **whole** converted value, including sign and decimal point. Since PHP 8.0 it can be `*`, taking the width from an extra argument. - **.precision**: for `e`, `E`, `f`, `F`, the number of digits after the point (6 when omitted); for `s`, a cutoff, the maximum number of characters kept. - **specifier**: the conversion itself. ## The specifiers you use every day | Specifier | Treats the value as | Example | Result | |---|---|---|---| | `d` | signed integer | `sprintf('%05d', 42)` | `00042` | | `u` | unsigned integer | `sprintf('%u', 42)` | `42` | | `s` | string | `sprintf('%-6s|', 'Net')` | `Net |` | | `f` | float, locale-aware point | `sprintf('%.2f', 119.9)` | `119.90` in the C locale | | `F` | float, always a `.` | `sprintf('%8.2F', 119.9)` | ` 119.90` | | `x` / `X` | integer in hex | `sprintf('%04X', 255)` | `00FF` | | `b` | integer in binary | `sprintf('%b', 5)` | `101` | | `e` | scientific notation | `sprintf('%.1e', 1200)` | `1.2e+3` | | `%` | a literal percent sign | `sprintf('%d%%', 20)` | `20%` | The difference between `f` and `F` matters for machine-readable output: `f` uses the decimal point of the current numeric locale, `F` never does. ## Formatting an invoice line An accounting export typically needs fixed-width, zero-padded identifiers and amounts with a fixed number of decimals: 1. **Invoice number**: `sprintf('INV-%d-%05d', $year, $sequence)` turns `2026` and `42` into `INV-2026-00042`. 2. **Amount column**: `sprintf('%10.2F', $net)` right-aligns the amount in ten characters with two decimals. 3. **Label column**: `sprintf('%-12s', $label)` left-aligns a label and pads it with spaces on the right. 4. **Filler**: `sprintf("%'.20s", $total)` pads with dots, a common look for printed totals. 5. **Reordering**: `sprintf('%2$s %1$s', 'EUR', '119.90')` prints `119.90 EUR`; translators can move arguments without changing the calling code. Remember that width counts every character: `sprintf('%06.2f', 3.14159)` is `003.14`, six characters including the point and both decimals, not six digits before the point. ## Traps - **`%d` truncates floats.** The value is converted to an integer first, so `sprintf('%d', 19.99)` gives `19`, not `20`. Format amounts with `f`/`F`, or keep them as integer minor units. - **Precision on `s` cuts.** `sprintf('%.3s', 'INVOICE')` returns `INV`; it does not pad. - **Too few values.** Since PHP 8.0 a format that needs more values than you pass throws `ArgumentCountError`; before 8.0 it returned `false` with a warning. An `argnum` of zero throws `ValueError`. - **A literal percent sign** must be written `%%`; a lone `%` starts a specification. ## The rest of the family - `printf()` writes the result to output and returns its **length** as an `int`. - `vsprintf()` and `vprintf()` take the values as one **array**, handy when they are already collected. - `fprintf()` writes to a stream resource, for example an open export file. Since PHP 8.4, a `sprintf()` call that uses only `%s` and `%d` is compiled into the equivalent string interpolation, so simple formats cost no more than concatenation. For amounts shown to people, `number_format()` adds thousands separators, which `sprintf()` never does. ## What interviewers listen for A good answer reads the prototype aloud and applies it to a concrete line rather than listing specifiers from memory. Expect probing on three points: - **Width versus precision**: width is a minimum for the whole value, precision is decimals for floats and a cutoff for strings. - **Integer conversion**: `%d` on a money amount truncates, which is a silent data bug rather than a crash. - **Failure mode**: in PHP 8 a missing value is an `ArgumentCountError`, so a broken format fails loudly in tests instead of writing `false` into an export file. Mentioning that `F` exists because `f` is locale-aware shows you have shipped files that other systems parse.

What is the difference between sprintf(), printf() and vsprintf() in PHP?
`sprintf()` returns the formatted string. `printf()` writes it to output and returns the number of bytes written. `vsprintf()` behaves like `sprintf()` but takes the values as a single array, which is convenient when they are already collected, for example from a database row.
Why does sprintf('%06.2f', 3.14159) return '003.14' rather than '000003.14'?
The width is the minimum length of the whole converted value, counting the digits on both sides and the decimal point. `3.14` is four characters, so two zeros bring it to six. To get more leading digits, increase the width to cover the decimals too.
How do argnum specifiers such as %1$s help when the format string is translated?
They select values by position instead of by order of appearance, so a translated format can place the amount before the currency or reuse a value twice without changing the PHP call. A format may use `%1$s` several times; the value is fetched once per use.

saying these in an interview costs you the question

sprintf('%d', 19.99) rounds the value to 20.
The width in %06.2f counts only the digits before the decimal point.
sprintf returns false when too few values are passed in PHP 8.
A precision on %s pads the string to that length.
printf returns the formatted string, just like sprintf.
%f and %F are interchangeable in every environment.

In PHP, how do sprintf() conversion specifications like %05d, %'*10s, %.2f and %1$s work when formatting invoice lines?

§ answer

basics

solid answer

code

§ go deeper

▾level notes4 levels

▾deep dive~4 min

The shape of a conversion specification

The specifiers you use every day

Formatting an invoice line

Traps

The rest of the family

What interviewers listen for

▾common follow-ups3 questions

▾red flags6 flags

The shape of a conversion specification

The specifiers you use every day

Formatting an invoice line

Traps

The rest of the family

What interviewers listen for

answer

go deeper