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PHP has a vast set of built-in functions that every developer should know to write efficient and secure code. Here are some essential ones:

1. String Functions

• strlen($string): Returns the length of a string.
• strpos($haystack, $needle): Finds the position of the first occurrence of a substring.
• str_replace($search, $replace, $subject): Replaces occurrences of a string within another string.
• substr($string, $start, $length): Extracts a portion of a string.
• trim($string): Removes whitespace (or other characters) from the beginning and end of a string.

2. Array Functions

• count($array): Returns the number of elements in an array.
• array_merge($array1, $array2): Merges two or more arrays.
• array_push($array, $value): Adds an element to the end of an array.
• array_pop($array): Removes and returns the last element of an array.
• in_array($value, $array): Checks if a value exists in an array.
• array_keys($array): Returns all the keys from an array.
• array_values($array): Returns all the values from an array.

3. File Handling Functions

• file_get_contents($filename): Reads the entire file into a string.
• file_put_contents($filename, $data): Writes data to a file.
• fopen($filename, $mode): Opens a file for reading or writing.
• fclose($handle): Closes an open file handle.
• fwrite($handle, $data): Writes data to an open file handle.

4. Date & Time Functions

• date($format): Formats a date/time string.
• strtotime($string): Converts a string to a Unix timestamp.
• time(): Returns the current Unix timestamp.
• mktime($hour, $minute, $second, $month, $day, $year): Creates a timestamp from given date and time.

5. Math Functions

• abs($number): Returns the absolute value of a number.
• ceil($number): Rounds a number up to the nearest integer.
• floor($number): Rounds a number down to the nearest integer.
• round($number, $precision): Rounds a number to a specified precision.
• rand($min, $max): Generates a random number.

6. Security & Validation Functions

• htmlspecialchars($string): Converts special characters to HTML entities to prevent XSS.
• md5($string): Generates an MD5 hash of a string (not recommended for security).
• password_hash($password, PASSWORD_DEFAULT): Hashes a password securely.
• password_verify($password, $hash): Verifies a hashed password.
• filter_var($data, FILTER_VALIDATE_EMAIL): Validates an email address.

7. Database Functions (MySQLi & PDO)

• mysqli_connect($host, $user, $password, $dbname): Connects to a MySQL database.
• mysqli_query($conn, $query): Executes a query on the database.
• mysqli_fetch_assoc($result): Fetches a result row as an associative array.
• PDO::prepare($query): Prepares a SQL query for execution with parameter binding.
• PDO::execute(): Executes a prepared statement.

8. JSON Handling

• json_encode($data): Converts an array or object into a JSON string.
• json_decode($json, true): Converts a JSON string into an array or object.

9. Error Handling

• error_log($message): Logs an error message.
• set_error_handler($callback): Defines a custom error handler.
• try { } catch (Exception $e) { }: Catches exceptions in a try block.

10. Miscellaneous

• die($message): Stops script execution and prints a message.
• exit($message): Stops script execution (similar to die()).
• isset($variable): Checks if a variable is set.
• empty($variable): Checks if a variable is empty.
• define('CONSTANT', 'value'): Defines a constant.

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