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Scoping rules (of variables inside a function)

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• Review: scoping rules
  • Scoping rules:

    Scoping rules = rules that state where in the program a variable (name) is usable (= accessible)

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• Scoping Rules of PHP
  • Scopes in PHP:

    There is only 2 scopes in a PHP program: Global (= program) scope Functon scope

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  • PHP scoping rules:

    A variable created (= first assigned) inside the main program has the global (= program) scope A global variable is accessible from everywhere in the main program But only after the variable was defined !!!         A global variable is also accessible inside any function: But only when the variable is defined a global clause !!!!         A variable created (= first assigned) in a function has the function scope A variable in a function scope can only be used inside the function where the variable was defined A variable in a function scope is a different variable than a global variable with the same name

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    Example: global scope and function scope

    <?php $a = 1; # Global var a print("Main: a = " . $a . "\n"); f( ); print("Main: a = " . $a . "\n"); function f( ) { print("f before: a = " . $a . "\n"); # Function scoped var a -- error !! $a = 4444; # Define function scoped var a print("f before: a = " . $a . "\n"); # Function scoped var a -- no error } ?>

    Output:

    Main: a = 1 PHP Notice: Undefined variable: a in scoping1.php on line 11 f before: a = <---- this a has not be created ! f before: a = 4444 Main: a = 1

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  • Example Program: (Demo above code)                                                

    How to run the program:

    ssh -X netid@holland php /home/cs377001/PHP/intro-PHP/scoping1.php

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• Accessing global variables in a function

  • How to access a global variable inside a function:

    We must first declare (= "pull in") a global variable before using it inside a function with the global clause: global $varName;

  • Example: accessing a global variable inside a function

    <?php $a = 1; # Global a print("Main: a = " . $a . "\n"); f($a); print("Main: a = " . $a . "\n"); /* ============================================================ function f( ) ============================================================ */ function f( ) { global $a; # ******** a will now access a global variable print("f before: a = " . $a . "\n"); # Uses global scoped var a !!! $a = 4444; print("f after: a = " . $a . "\n"); # Uses global scoped var a !!! } print("Main: a = " . $a . "\n"); ?>

    Output:

    Main: a = 1 f before: a = 1 f before: a = 4444 Main: a = 4444 <-- Global variable is updated !

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  • Example Program: (Demo above code)                                                

    How to run the program:

    ssh -X netid@holland php /home/cs377001/PHP/intro-PHP/scoping1.php