substr() returns the portion of string specified by the start and length parameters.
If start is non-negative, the returned string will start at the start'th position in string, counting from zero. For instance, in the string 'abcdef', the character at position 0 is 'a', the character at position 2 is 'c', and so forth.
Example 1. Basic substr() usage
If start is negative, the returned string will start at the start'th character from the end of string.
If length is given and is positive, the string returned will contain at most length characters beginning from start (depending on the length of string). If string is less than start characters long, FALSE will be returned.
If length is given and is negative, then that many characters will be omitted from the end of string (after the start position has been calculated when a start is negative). If start denotes a position beyond this truncation, an empty string will be returned.