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# Begin of assignment
substr(        # Start of substr call, using substr as lvalue
...
'';            # Righthand operand of the assignment operator
# End of assignment. An assignment returns the assigned value,
# '' in this case.

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