Writing to the return value of a function in PHP

user_9109145 0
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The PHP Language Specification says:

A variable is an expression that can in principle be used as an lvalue

and

The value of a function call is a modifiable lvalue only if the function returns a modifiable value byRef.

and the grammar from zend_language_parser.y

expr:
   variable T_CONCAT_EQUAL expr

callable_variable:
        simple_variable
            { $$ = zend_ast_create(ZEND_AST_VAR, $1); }
    |   dereferencable '[' optional_expr ']'
            { $$ = zend_ast_create(ZEND_AST_DIM, $1, $3); }
    |   constant '[' optional_expr ']'
            { $$ = zend_ast_create(ZEND_AST_DIM, $1, $3); }
    |   dereferencable '{' expr '}'
            { $$ = zend_ast_create_ex(ZEND_AST_DIM, ZEND_DIM_ALTERNATIVE_SYNTAX, $1, $3); }
    |   dereferencable T_OBJECT_OPERATOR property_name argument_list
            { $$ = zend_ast_create(ZEND_AST_METHOD_CALL, $1, $3, $4); }
    |   function_call { $$ = $1; }
;

variable:
        callable_variable
            { $$ = $1; }
    |   static_member
            { $$ = $1; }
    |   dereferencable T_OBJECT_OPERATOR property_name
            { $$ = zend_ast_create(ZEND_AST_PROP, $1, $3); }
;

So why can't I do this in PHP 7.3?

<?php
$a = 'HELLO';
function &foo() { 
    global $a;
    return $a;
}

foo() .= ' WORLD';
echo $a;

PHP Fatal error:  Can't use function return value in write context in ...

To follow on from this question, with reference to the grammar above:

When is it ok to have a function call on the left hand side of a simple/compound assignment expression?

php
1 Answers

While you can return reference from function you can't write into that reference directly.

You have to assign the reference first.

$a = 'HELLO';
function &foo() { 
    global $a;
    return $a;
}

$b =& foo();
$b .= ' WORLD';
echo $a;
user_119770680
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