Whitespace, it can make all the difference in the world when attempting to determine if a string is empty or not. Anytime you are working with user input I recommend stripping whitespace from both sides of the string. When stripping whitespace you can choose to strip from both sides or explicitly the whitespace at the beginning or end of the string.
PHP’s definition of whitespace more than just ordinary spaces, it also includes tabs
\t, new lines
\n, carriage returns
\r, the NUL-byte
\0 and the vertical tab