PHP provides a rich set of built-in functions that cover a wide range of functionalities, from string manipulation and array handling to date and time operations and mathematical calculations. These built-in functions are readily available and do not require any additional code to use. Here’s an overview of some common categories of built-in functions in PHP:

1. String Functions

PHP offers numerous functions for manipulating strings. These functions are used to perform operations such as searching, replacing, and formatting strings.

Examples:

• strlen(): Returns the length of a string.

  echo strlen("Hello, World!");  // Outputs: 13
  

• strtoupper(): Converts a string to uppercase.

  echo strtoupper("hello");  // Outputs: HELLO
  

• strpos(): Finds the position of the first occurrence of a substring in a string.

  echo strpos("Hello, World!", "World");  // Outputs: 7
  

• substr(): Returns a part of a string.

  echo substr("Hello, World!", 7, 5);  // Outputs: World
  

2. Array Functions

Array functions allow you to perform various operations on arrays, such as sorting, filtering, and merging.

Examples:

• count(): Counts all elements in an array.

  $array = array(1, 2, 3);
  echo count($array);  // Outputs: 3
  

• array_merge(): Merges one or more arrays into one.

  $array1 = array("a", "b");
  $array2 = array("c", "d");
  $result = array_merge($array1, $array2);
  print_r($result);  // Outputs: Array ( [0] => a [1] => b [2] => c [3] => d )
  

• array_search(): Searches for a value in an array and returns the key if found.

  $array = array("a" => "apple", "b" => "banana");
  echo array_search("banana", $array);  // Outputs: b
  

• sort(): Sorts an array in ascending order.

  $array = array(3, 1, 2);
  sort($array);
  print_r($array);  // Outputs: Array ( [0] => 1 [1] => 2 [2] => 3 )
  

3. Mathematical Functions

Mathematical functions are used to perform various mathematical operations and calculations.

Examples:

• abs(): Returns the absolute value of a number.

  echo abs(-10);  // Outputs: 10
  

• round(): Rounds a floating-point number to the nearest integer.

  echo round(3.6);  // Outputs: 4
  

• max(): Finds the highest value in a list of arguments.

  echo max(1, 5, 3);  // Outputs: 5
  

• min(): Finds the lowest value in a list of arguments.

  echo min(1, 5, 3);  // Outputs: 1
  

4. Date and Time Functions

These functions allow you to work with date and time values, including formatting and calculations.

Examples:

• date(): Formats a local date and time.

  echo date("Y-m-d");  // Outputs: 2024-09-03 (current date)
  

• time(): Returns the current Unix timestamp.

  echo time();  // Outputs: 1693687432 (current timestamp)
  

• strtotime(): Parses an English textual datetime description into a Unix timestamp.

  echo strtotime("next Monday");  // Outputs: timestamp for the next Monday
  

• mktime(): Get the Unix timestamp for a date.

  echo mktime(0, 0, 0, 9, 3, 2024);  // Outputs: timestamp for 2024-09-03
  

5. File Handling Functions

File handling functions allow you to perform operations such as reading from and writing to files.

Examples:

• fopen(): Opens a file or URL.

  $file = fopen("example.txt", "r");
  

• fread(): Reads a file.

  $content = fread($file, filesize("example.txt"));
  

• fwrite(): Writes to a file.

  fwrite($file, "Hello, World!");
  

• fclose(): Closes an open file.

  fclose($file);
  

6. Utility Functions

Utility functions provide various functionalities such as data type checking and debugging.

Examples:

• is_array(): Checks if a variable is an array.

  $array = array(1, 2, 3);
  echo is_array($array);  // Outputs: 1 (true)
  

• isset(): Checks if a variable is set and is not null.

  $var = "Hello";
  echo isset($var);  // Outputs: 1 (true)
  

• empty(): Checks if a variable is empty.

  $var = "";
  echo empty($var);  // Outputs: 1 (true)
  

• print_r(): Prints human-readable information about a variable.

  $array = array("a" => "apple", "b" => "banana");
  print_r($array);