strstr() function, the function searches for the first occurrence of a string in another string
<?php
echo strstr("I love Shanghai!","Shanghai");
?>
The output is shanghai, the function is case-sensitive, and stristr() has the same effect as case-insensitive.
$_FILES is a global array, file uploads are made using
$_FILES["file"]["name"] - the name of the uploaded file
$_FILES["file"]["type"] - the type of the uploaded file
$_FILES["file"]["size"] - the size of the uploaded file in bytes
$_FILES["file"][" tmp_name"] - the name of the temporary copy of the file stored on the server
$_FILES["file"]["error"] - the error code caused by the file upload
eg:$_FILES["file"]["type"] == "image/jpeg" to limit the file type.