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How does the vfprintf function support formatted content output of multiple different data types? Practical examples

gitbox 2025-05-29

What is vfprintf()?

vfprintf() is a function provided by PHP to write formatted strings into a specified file stream. Unlike fprintf() , vfprintf() receives an array of parameter lists. This makes it more flexible when dynamically splicing and processing multiple types of data.

Function definition:

 int vfprintf ( resource $handle , string $format , array $args )

  • $handle : File pointer resource, usually returned by fopen() .

  • $format : Format string.

  • $args : The array of data to be inserted in the formatted string.

Data types supported by format strings

In vfprintf() , we can use a series of format identifiers to support different types of output. Common ones include:

  • %s : string

  • %d : integer (decimal)

  • %f : floating point number

  • %x : hexadecimal (lowercase)

  • %X : Hexadecimal (caps)

  • %b : binary

  • %c : ASCII character

These identifiers can be combined with modifiers (such as precision, padding, etc.) to complete more complex output requirements.

Practical examples

Example 1: Write strings and integers

 <?php
$fp = fopen("log.txt", "a");
$name = "Alice";
$age = 30;
vfprintf($fp, "Name: %s, Age: %d\n", array($name, $age));
fclose($fp);
?>

This code writes the following to the log.txt file:

 Name: Alice, Age: 30

Example 2: Write a floating point number and keep two decimal places

 <?php
$fp = fopen("log.txt", "a");
$product = "Coffee";
$price = 19.456;
vfprintf($fp, "Product: %s, Price: \$%.2f\n", array($product, $price));
fclose($fp);
?>

Output:

 Product: Coffee, Price: $19.46

%.2f here means keeping two decimal places.

Example 3: Dynamically generate URLs and write files

 <?php
$fp = fopen("links.txt", "a");
$endpoint = "api/data";
$id = 42;
$url = "https://gitbox.net/%s?id=%d";
vfprintf($fp, $url."\n", array($endpoint, $id));
fclose($fp);
?>

The contents written to the file are:

 https://gitbox.net/api/data?id=42

Example 4: Mixed type writing log

 <?php
$fp = fopen("debug.log", "a");
$time = date("Y-m-d H:i:s");
$userId = 101;
$success = true;
$ip = "192.168.1.10";
vfprintf($fp, "[%s] User ID: %d, Success: %s, IP: %s\n", array(
    $time, $userId, $success ? "true" : "false", $ip
));
fclose($fp);
?>

The output is similar:

 [2025-05-29 15:30:00] User ID: 101, Success: true, IP: 192.168.1.10

Tips and precautions

  1. Security : Avoid user input being spliced ​​directly into format strings, preventing format injection attacks.

  2. The order of the array must be accurate : the parameter array must correspond to the format string one by one, and the order cannot be wrong.

  3. File permissions : Make sure that the file written has write permission, otherwise it will cause fopen() or vfprintf() to fail.

  4. Multilingual adaptation : When formatting the output, you can pre-process it with sprintf() and vprintf() .

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