To check if a file exists you use the PHP function file_exists. Input the path to the file and it will return TRUE if the file exists otherwise FALSE.

if (file_exists("assets/images/smile_face.jpg")) {
//file exists
} else {
//no file found
}

Incredibly simple and certainly of use.

file_exists also works for checking if a directory exists

if (file_exists("assets/images/")) {
//exists
} else {
//folder not found
}

If you need to create a directory or folder in PHP use mkdir.