What are common methods for manipulating strings in PHP?

Common methods for manipulating strings in PHP include functions like `strlen()` to get the length of a string, `strtolower()` and `strtoupper()` to convert a string to lowercase or uppercase, `str_replace()` to replace occurrences of a substring within a string, `substr()` to extract a part of a string, and `trim()` to remove whitespace from the beginning and end of a string.

// Example of using common string manipulation functions in PHP
$string = "Hello, World!";
echo strlen($string); // Output: 13
echo strtolower($string); // Output: hello, world!
echo strtoupper($string); // Output: HELLO, WORLD!
echo str_replace("Hello", "Hi", $string); // Output: Hi, World!
echo substr($string, 7); // Output: World!
echo trim("  Hello, World!  "); // Output: Hello, World!