Here’s a list of common string functions in PHP:

1. strlen() - Returns the length of a string.

   strlen("Hello World"); // 11
   

2. str_word_count() - Counts the number of words in a string.

   str_word_count("Hello World"); // 2
   

3. strpos() - Finds the position of the first occurrence of a substring.

   strpos("Hello World", "World"); // 6
   

4. str_replace() - Replaces some characters in a string.

   str_replace("World", "PHP", "Hello World"); // Hello PHP
   

5. substr() - Returns part of a string.

   substr("Hello World", 6); // World
   

6. trim() - Removes whitespace or other characters from the beginning and end of a string.

   trim("  Hello World  "); // "Hello World"
   

7. strtolower() - Converts a string to lowercase.

   strtolower("HELLO WORLD"); // hello world
   

8. strtoupper() - Converts a string to uppercase.

   strtoupper("hello world"); // HELLO WORLD
   

9. ucfirst() - Capitalizes the first letter of a string.

   ucfirst("hello world"); // Hello world
   

10. lcfirst() - Converts the first letter of a string to lowercase.

    lcfirst("Hello World"); // hello World
    

11. ucwords() - Capitalizes the first letter of each word in a string.

    ucwords("hello world"); // Hello World
    

12. md5() - Calculates the MD5 hash of a string.

    md5("hello"); // 5d41402abc4b2a76b9719d911017c592
    

13. sha1() - Calculates the SHA-1 hash of a string.

    sha1("hello"); // aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d
    

14. addslashes() - Escapes a string with backslashes.

    addslashes("O'Reilly"); // O\'Reilly
    

15. htmlspecialchars() - Converts special characters to HTML entities.

    htmlspecialchars("<a href='test'>Test</a>"); // &lt;a href=&#039;test&#039;&gt;Test&lt;/a&gt;
    

16. str_repeat() - Repeats a string a specified number of times.

    str_repeat("Hello", 3); // HelloHelloHello
    

17. explode() - Splits a string by a string and returns an array.

    explode(" ", "Hello World"); // ["Hello", "World"]
    

18. implode() (alias of join()) - Joins array elements with a string.

    implode("-", ["Hello", "World"]); // Hello-World
    

19. strrev() - Reverses a string.

    strrev("Hello"); // olleH
    

20. similar_text() - Calculates the similarity between two strings.

    similar_text("Hello", "Hell"); // 80 (percent similarity)
    

21. number_format() - Formats a number with grouped thousands.

    number_format(1000000); // 1,000,000
    

22. ord() - Returns the ASCII value of the first character of a string.

    ord("A"); // 65
    

23. chr() - Returns a character from a specified ASCII value.

    chr(65); // A
    

24. parse_str() - Parses a query string into variables.

    parse_str("name=John&age=25", $output);
    // $output = ["name" => "John", "age" => "25"]
    

25. sprintf() - Returns a formatted string.

    sprintf("There are %d apples", 5); // There are 5 apples
    

These functions are widely used for string manipulation in PHP. Let me know if you need examples or explanations for any specific ones!