How can placeholders or variables be effectively used in PHP string manipulation functions to handle variable content?

When dealing with variable content in PHP string manipulation functions, it is essential to use placeholders or variables to dynamically insert the variable content into the strings. This approach allows for easier readability, maintainability, and flexibility in handling different types of content within the strings.

$name = "John";
$age = 30;

// Using placeholders to insert variables into a string
$message = "Hello, my name is $name and I am $age years old.";
echo $message;