Essential Books

Assign by Reference

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This is the first phase of referencing. Essentially when you assign by reference, you’re allowing two variables to share the same value as such. 

$foo = &$bar;

$foo and $bar are equal here. They do not point to one another. They point to the same place (the “value”).

You can also assign by reference within the array() language construct. While not strictly being an assignment by reference. 

$foo = 'hi';
$bar = array(1, 2);
$array = array(&$foo, &$bar[0]);
Note, however, that references inside arrays are potentially dangerous. Doing a normal (not by reference) assignment with a reference on the right side does not turn the left side into a reference, but references inside arrays are preserved in these normal assignments. This also applies to function calls where the array is passed by value.

Assigning by reference is not only limited to variables and arrays, they are also present for functions and all “pass-by-reference” associations. 

function incrementArray(&$arr) {
    foreach ($arr as &$val) {
        $val++;
    }
}

function &getArray() {
    static $arr = [1, 2, 3];
    return $arr;
}

incrementArray(getArray());
var_dump(getArray()); // prints an array [2, 3, 4]

Assignment is key within the function definition as above. You can not pass an expression by reference, only a value/variable. Hence the instantiation of $a in bar().

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References:
* References
* Assign by Reference
* Return by Reference
* Pass by Reference
Table Of Contents
0 Getting started
1 Variables
2 Arrays
3 Functional programming
4 Types
5 Autoloading primer
6 Exception handling and error reporting
7 Working with dates and time
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18 Dependency injection
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24 Composer dependency manager
25 Magic methods
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27 File handling
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29 Type hinting
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35 UTF-8
36 URLs
37 Object serialization
38 PHPDoc
39 Contributing to PHP Manual
40 String parsing
41 Loops
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46 Type juggling and non-strict comparison issues
47 Security
48 PHP MySQLi
49 Command Line Interface CLI
50 Localization
51 Debugging
52 Superglobal variables
53 Unit testing
54 Variable scope
55 References
56 Compilation errors and warning
57 Installing PHP on Windows
58 DateTime class
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60 Performance
61 Common Errors
62 Installing on Linux / Unix
63 Contributing to PHP Core
64 Coding conventions
65 Using MongoDB
66 Asynchronous programming
67 Using SQLSRV
68 Unicode Support
69 Functions
70 Create PDF files
71 Hot to detect client IP address
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73 Image processing with GD
74 Multiprocessing
75 SOAP Server
76 Machine learning
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80 Cryptography
81 PDO
82 SQLite3
83 Sockets
84 Outputting the value of a variable
85 String formatting
86 Compile PHP extensions
87 MongoDB
88 Manipulating an array
89 Executing upon an array
90 Processing multiple arrays together
91 SPL data structures
92 Comments
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95 Imagick
96 SimplXML
97 HTTP Authentication
98 Recipies
99 BC Math Binary Calculator
100 Docker deployment
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102 APCu
103 Design patterns
104 Secure remember me
105 PHP mysqli
106 PHP built-in server
107 How to break down an URL
108 PSR
109 Contributors