<aside> 💡 To learn WordPress theme and plugin development, the following PHP stuff would be more than sufficient. This guideline is provided by Kamal Ahmed, who has worked for WP Developers, rtCamps, UberSuggest, SiteCare, and other companies.

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1. Variables & Data Types: (string, integer, float, array, object). Minimum 15 array function & 10 string function.
2. Functions and Scopes.
3. Loops (for, foreach, while) & Conditions (if, else, switch, try catch).
4. Working with FORMS: get, post, delete, put difference. Validating and Escaping user submitted data.
5. Global variables: GET, POST, REQUEST differences and usage.
6. Include and Require
7. For Object Oriented PHP:
   1. Class,
   2. Object,
   3. Class property,
   4. Class method,
   5. Visibility,
   6. Static,
   7. Inheritance
8. A little intermediate or relatively less common topics but very important and found sometimes in plugins and themes.
   1. Interface, traits, Singleton, Magic Methods, Namespace etc.
   2. Output Buffering, SESSION, COOKIES, Working with APIs using curl. Date and Times,
   3. Working with files (opening, reading, writing etc)

The most commonly used Functions for manipulating Arrays in PHP.

Please see the documentation for example and details understanding.

1. sizeof($array)

   // Use this function to find out how many elements an array contains

2. array_values($array)

   //Use this function to retrieve all the values from an associative array.

3. array_keys($array)

   // Use this function to retrieve all the keys from an associative array.

4. array_pop($array)

   //Use this function to remove an element from the end of an array.

5. array_push($array, $value)

   // This function adds an element to the end of an array.

6. array_shift($array)

   //This function removes an element from the beginning of an array.

7. array_unshift($array, $value)

   // This function adds an element to the beginning of an array.

8. array_map($callback, $array1)

   //This function returns an array containing all the elements of array1 after applying the callback function to each one. You can pass more than one array. The number of parameters that the callback function accepts should match the number of arrays passed to the array_map()