Beginning PHP and PostgreSQL 8: From Novice to Professional
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Over the years Jason has been published more than times within popular publications such as Developer. Jason is cofounder of the wildly popular CodeMash Conference, the largest multi-day developer event in the Midwest. Produktinformationen Format: PDF. Kopierschutz: AdobeDRM.
Seitenzahl: Erscheinungsdatum: Sprache: Englisch. Therefore, in the last line of the previous listing, an object of class appliance named blender is created. Chapters 6 and 7 are devoted to thorough coverage of this important feature. Special Datatypes Special datatypes encompass those types serving some sort of niche purpose, which makes it impossible to group them in any other type category. The resource and null datatypes fall under this category. Resource PHP is often used to interact with some external data source: databases, files, and network streams all come to mind.
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Typically this interaction takes place through handles, which are named at the time a connection to that resource is successfully initiated. These handles are of the resource datatype. Not all functions return resources; only those that are responsible for binding a resource to a variable found within the PHP script do. Null does not mean blank space, nor does it mean zero; it means no value, or nothing. The null datatype recognizes only one value, Null: Type Casting Forcing a variable to behave as a type other than the one originally intended for it is known as type casting.
A variable can be evaluated once as a different type by casting it to another. This is accomplished by placing the intended type in front of the variable to be cast. A type can be cast by inserting one of the casts shown in Table in front of the variable. Now consider the opposite scenario. Note that the double will be rounded down every time, regardless of the decimal value. You can also cast a datatype to be a member of an array. If it is cast against an existing array, that array will be wiped out, leaving only the newly cast value in the first position.
What happens if you cast a string datatype to that of an integer? How about the opposite procedure, casting an integer to a string? One final example: any datatype can be cast as an object. However, if it begins with anything other than a numerical representation, the value is zero. Consider another example: In this example, a string is converted to Boolean type in order to evaluate the if statement. Consider one last, particularly interesting, example. If a string used in a mathematical calculation includes a.
Seven possible type values are available: array, boolean, float, integer, null, object, and string.
Beginning PHP and PostgreSQL 8 (eBook) | ALDI life
In total, eight possible return values are available: array, boolean, double, integer, object, resource, string, and unknown type. Because all of these functions follow the same naming convention, arguments, and return values, their introduction is consolidated to a single general form, presented here. Each determines whether a variable, specified by var, satisfies a particular condition specified by the function name. Delimiting the dollar sign with a backslash will accomplish this.
Identifiers Identifier is a general term applied to variables, functions, and various other user-defined objects. Furthermore, identifiers can consist of only letters, numbers, underscore characters, and other ASCII characters from through You can find a complete list of these keywords in the PHP manual appendix.
Beginning PHP and PostgreSQL 8
Variables Although variables have been used within numerous examples found in this chapter, the concept has yet to be formally introduced. This section does so, starting with a definition. Simply put, a variable is a symbol that can store different values at different times.
For example, suppose you create a Web-based calculator capable of performing mathematical tasks. Of course, the user will want to plug in values of his choosing; therefore, the program must be able to dynamically store those values and perform calculations accordingly. At the same time, the programmer requires a user-friendly means for referring to these value-holders within the application. The variable accomplishes both tasks. Given the importance of this programming concept, it would be wise to explicitly lay the groundwork as to how variables are declared and manipulated.
In this section, these rules are examined in detail.
Variable names follow the same naming rules as identifiers. That is, a variable name can begin with either a letter or an underscore, and can consist of letters, underscores, numbers, or other ASCII characters ranging from through Rather, variables can be declared and assigned values simultaneously.
Good programming practice dictates that all variables should be declared prior to use, preferably with an accompanying comment. Two methodologies are available for variable assignment: by value and by reference. Both are introduced next. Value Assignment Assignment by value simply involves copying the value of the assigned expression to the variable assignee.
This is the most common type of assignment. Reference Assignment PHP 4 introduced the ability to assign variables by reference, which essentially means that you can create a variable that refers to the same content as another variable does. Therefore, a change to any variable referencing a particular item of variable content will be reflected among all other variables referencing that same content. Chapters 4 and 6 cover these features, respectively.
Variable Scope However you declare your variables by value or by reference , you can declare variables anywhere in a PHP script. The location of the declaration greatly influences the realm in which a variable can be accessed, however. This accessibility domain is known as its scope. That is, it can be referenced only in that function. Any assignment outside of that function will be considered to be an entirely different variable from the one contained in the function.
Note that when you exit the function in which a local variable has been declared, that variable and its corresponding value are destroyed.
W. Jason Gilmore
Local variables are helpful because they eliminate the possibility of unexpected side effects, which can result from globally accessible variables that are modified, intentionally or not. Although those arguments accept values that come from outside of the function, they are no longer accessible once the function has exited. Parameters passed by reference will indeed be affected by any changes made to the parameter from within the function. Function parameters are declared after the function name and inside parentheses. Global Variables In contrast to local variables, a global variable can be accessed in any part of the program.
To modify a global variable, however, it must be explicitly declared to be global in the function in which it is to be modified. This is accomplished, conveniently enough, by placing the keyword GLOBAL in front of the variable that should be recognized as global. Placing this keyword in front of an already existing variable tells PHP to use the variable having that name. This local declaration would be implicitly set to 0, and then incremented by 1 to display the value 1. Therefore, although global variables can be extremely useful, be prudent when using them. Static Variables The final type of variable scoping to discuss is known as static.
Therefore, the outcome is: 1 2 3 Static scoping is particularly useful for recursive functions. Recursive functions are a powerful programming concept in which a function repeatedly calls itself until a particular condition is met. Recursive functions are covered in detail in Chapter 4. PHP creates some of the variables, while the availability and value of many of the other variables are specific to the operating system and Web server.