What are common errors when using str_replace() and fclose() functions in PHP?

Common errors when using `str_replace()` in PHP include not assigning the replaced string back to a variable or using incorrect parameters. When using `fclose()`, common errors include trying to close a file that is not open or attempting to close a file handle multiple times. To solve these issues, make sure to properly assign the result of `str_replace()` to a variable and check if the file is open before closing it using `fclose()`.

// Example of correct usage of str_replace()
$string = "Hello, World!";
$replaced_string = str_replace("Hello", "Hi", $string);
echo $replaced_string;

// Example of correct usage of fclose()
$file = fopen("example.txt", "r");
if ($file) {
    fclose($file);
} else {
    echo "Error: File not open.";
}