# Begin of assignment
substr(        # Start of substr call, using substr as lvalue
    $string,   # First argument
    2,         # Second argument
    6          # Third argument
)
               # Anything after this is not part of the substr call
=              # Assignment operator
'';            # Righthand operand of the assignment operator
# End of assignment. An assignment returns the assigned value,
# '' in this case.

##</code><code>##

substr(        # Start of substr call, returning the old stuff
    $string,   # First argument
    2,         # Second argument
    6,         # Third argument
    ''         # Fourth argument: replacement text
);