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Cannot pass parameter %d by reference

Description

When a parameter is set to be passed by reference, there is a & character before its name. In that case, the argument must be a data container, such as a variable, a property, a static property, an array index, but it cannot be a literal value, a constant, or the result of an expression.

Example

<?php

function foo(&$a) {}

// invalid cases
foo(1);
foo(1 + 2);

const A = 1;
foo(A);

class X {
    const B = 2;
}
foo(X::B);

?>

Literal Examples

  • Cannot pass parameter 1 by reference

Alternatives

  • Store the value in a variable and passe the variable.
  • Remove the reference in the method signature.

In more recent PHP versions, this error message is now :ref:%s():-argument-#%d%s%s%s-cannot-be-passed-by-reference.