PHP Functions: User-Defined and System

• PHP functions are reusable code blocks that perform specific tasks.
• Utilizing functions helps reduce redundancy, creating cleaner and more efficient code.

Types of Functions

• User Defined Functions are custom functions created by the user.
• System Functions are built-in functions that are part of the PHP language.

User Defined Functions

• In addition to the built-in PHP functions, you can create your own functions.
• Functions consist of a block of statements for repeated use in a program.
• Functions do not automatically execute upon page load; they are executed through a function call.
• The syntax for declaring a user-defined function starts with the word "function", followed by the function name, parentheses, and curly braces enclosing the code to be executed.
  • function functionName() { code to be executed; }

Function Arguments

• Information can be passed to functions through arguments, which are like variables.
• Arguments are specified inside the parentheses after the function name, separated by commas.

Return Type Argument

• PHP 7 supports type declarations for the return statement.
• Enabling strict requirements with type declarations throws a "Fatal Error" on a type mismatch, similar to function arguments.
• To declare a type for the function return, add a colon (:) and the type before the opening curly bracket when declaring the function.

String Functions

• PHP string functions are built-in tools designed to manipulate text within PHP.
• They help with tasks such as finding string length, replacing parts, and converting text case.
• These functions simplify text handling and formatting in web applications.

Basic String Functions

• strlen(): Returns the length of a string.
• strtolower(): Converts a string to lowercase.
• strtoupper(): Converts a string to uppercase.
• ucwords(): Capitalizes the first letter of each word in a string.

Searching and Replacing Functions

• strpos(): Finds the position of the first occurrence of a substring in a string.
• str_replace(): Replaces all occurrences of a search string with a replacement string.
• substr(): Returns a part of a string.

Trimming Functions

• trim(): Removes whitespaces (or other characters) from both ends of a string.
• ltrim(): Removes whitespaces (or other characters) from the beginning of a string.
• rtrim(): Removes whitespaces (or other characters) from the end of a string.

Date and Time Codes

• Y: Represents a four-digit year.
• y: Represents a two-digit year.
• a: Lowercase AM or PM.
• A: Uppercase AM or PM.
• B: Swatch Internet time (000 to 999).
• g: 12-hour format of an hour (1 to 12).
• G: 24-hour format of an hour (0 to 23).
• h: 12-hour format of an hour (01 to 12).
• H: 24-hour format of an hour (00 to 23).
• i: Minutes with leading zeros (00 to 59).
• s: Seconds with leading zeros (00 to 59).
• u: Microseconds (added in PHP 5.2.2).
• e: The timezone identifier (e.g., UTC, GMT, Atlantic/Azores).
• I (capital i): Indicates if the date is in Daylight Savings Time (1 if yes, 0 if no).
• O: Difference to Greenwich Time (GMT) in hours (e.g., +0100).
• P: Difference to Greenwich Time (GMT) in hours:minutes (added in PHP 5.1.3).
• T: Timezone abbreviations (e.g., EST, MDT).
• Z: Timezone offset in seconds; negative for timezones west of UTC (-43200 to 50400).
• c: The ISO-8601 date format (e.g., 2013-05-05T16:34:42+00:00).
• r: The RFC 2822 formatted date (e.g., Fri, 12 Apr 2013 12:01:05 +0200).
• U: Seconds since the Unix Epoch (January 1, 1970 00:00:00 GMT).

Basic Date Functions

• date(): Returns the current date and time.
• time(): Returns the Unix timestamp.

Date Difference

• date_diff(): Calculates the difference between two dates.
  • Example:
    • $date1 = date_create("2023-01-01");
    • $date2 = date_create("2024-01-01");
    • $diff = date_diff($date1, $date2);
    • echo $diff->format("%R%a days"); // Output: +365 days

Adding and Subtracting Dates

• date_add(): Adds an interval to a date.
• date_sub(): Subtracts an interval from a date.
  • Example:
    • $date = date_create("2023-01-01");
    • date_add($date, date_interval_create_from_date_string("10 days"));
    • echo date_format($date, "Y-m-d"); // Output: 2023-01-11

Midterm Lab Activity 1

• Problem 1: Write a function factorial that takes a positive integer n and returns its factorial, which is the product of all positive integers less than or equal to n.
  • Example: echo factorial(5); // Output: 120
• Problem 2: Write a PHP script that takes a date in the format Y-m-d and calculates the number of days until the next occurrence of that date.
  • Example: echo daysUntilNextOccurrence('2024-10-11'); // Output: (number of days until next occurrence)
• Problem 3: Write a function reverseString that takes a string and returns the string reversed.
  • Example: echo reverseString("Hello, World!"); // Output: !dlroW ,olleH