If you specify a substring which is partly outside the string, the part within the string is returned. If the substring is totally outside the string a warning is produced.

You can use the substr() function as an lvalue, in which case EXPR must be an lvalue. If you assign something shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf().