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Variables (dynamic typing !!!)

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• Recall: PHP data types
  • Recall that PHP has 8 data types.

    Four scalar types: boolean integer float (C's double)           string Two compound types: array object (C's struct)           Two special types: resource (special variable, holding a reference to an external resource) NULL

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  • Recall: PHP uses Implicit variable definitions

    A variable is defined "implicitly" when: You assign a value to the variable           (That is a common practice in scripting languages - these programmers want speed) (The implicit variable definition make scripting languages not suitable for large programming projects)

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• The data type of a variable
  • Important fact:

    The data type of a variable in PHP is determined by: The type of the current value that was stored inside that variable !!!!           In other words: The type of a variable in PHP can change !!!         This feature is called dynamic typing and it's often found in scripting languages....

    Example:

    // gettype($a) return a string describing the data type of the variable $a <?php $a = 12; print ("a = " . $a . " Type of a = " . gettype($a) . "\n"); $a = 12.0; print ("a = " . $a . " Type of a = " . gettype($a) . "\n"); $a = "12"; print ("a = " . $a . " Type of a = " . gettype($a) . "\n"); $a = true; print ("a = " . $a . " Type of a = " . gettype($a) . "\n"); ?>

    Output:

    a = 12 Type of a = integer <-- The data type changes !!! a = 12 Type of a = double a = 12 Type of a = string a = 1 Type of a = boolean

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  • Example Program: (Demo above code)                                                

    How to run the program:

    ssh -X yourID@holland php /home/cs377001/PHP/intro-PHP/gettype1.php

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• Comment: Dynamic Typing in scripting languages
  • Dynamic typing:

    The data type of a (same) variable can change during the execution of a program

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  • Scripting languages and dynamic typing:

    The dynamic typeing feature is often found in scripting programming languages (e.g.: PERL ) Since PHP is a scripting language , it's not a surpise that it has dynamic typing....

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  • NOTE:

    Do not get too "wild" with the dynamic typing feature (you don't need to) You will do yourself a great favor if you do not assign different type of values to the same variable at different times...