How can PHP developers ensure compatibility with different PHP versions when using string manipulation functions?

PHP developers can ensure compatibility with different PHP versions when using string manipulation functions by checking the PHP version before using certain functions. They can use the `function_exists()` function to check if a specific function is available in the current PHP version and then provide an alternative solution if the function is not available.

// Check if the function exists before using it
if (function_exists('mb_strtoupper')) {
    $upperCaseString = mb_strtoupper($string);
} else {
    $upperCaseString = strtoupper($string);
}

// Alternative solution for converting a string to uppercase if mb_strtoupper is not available
if (function_exists('mb_convert_case')) {
    $upperCaseString = mb_convert_case($string, MB_CASE_UPPER, 'UTF-8');
} else {
    $upperCaseString = strtoupper($string);
}