PHP Reading notes (3)-constants

Last Update:2016-06-29 Source: Internet

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What is a constant What is a constant? Constants can be understood as values that do not change, or when a constant value is defined, it cannot be changed anywhere else in the script. This is how the manual defines PHP constants: constants are identifiers (names) of simple Values. As the name implies, the value cannot be changed during script execution (except for the so-called magic constants, they are not constants). Changshime is considered to be case Sensitive. traditionally, constant identifiers are always capitalized. the constant name and any other PHP tags follow the same naming conventions. A valid constant name begins with a letter or underscore followed by any letter, number, or underscore. This is expressed in regular expressions:[a-za-z_\x7f-\xff][a-za-z0-9_\x7f-\xff]*. Constants in PHP are divided into custom constants and system constants . A custom constant is a constant defined according to the needs of our development, and it is defined by using the function define () in Php.After PHP 5.3.0, You can use the const keyword to define constants outside of the class Definition. Once a constant is defined, it can no longer be changed or Undefined.Constants can only contain scalar data (boolean,integer,float, and string). You can define resource constants, but avoid them as much as possible, because they cause unpredictable results. You can simply get the value of a constant by specifying its name, and unlike a variable, you should not precede the constant with the $ symbol. If the constant name is dynamic, you can also use the function constant () to get the value of the Constant. A list of all defined constants can be obtained with get_defined_constants (). the second parameter, "value," is a required parameter, which is the value of the Constant. The third parameter, "case_sensitive", is an optional parameter, specifying whether it is case sensitive, set to True to be insensitive, and generally without specifying a third parameter, the value of the default third parameter is False.(note: string indicates that the parameter type is a string type, mixed indicates that the parameter type can be accepted as a number of different types, Case_sensitive = True indicates that the default is Boolean type True) What is the use of constants? The main function of a constant is to avoid duplicate definitions and tamper with variable Values. When we are working on Team development, or when the code is very large, for some quantities that do not change after the first definition, if we use variables, without knowing the same variable name, the value of the variable will be replaced, which will cause the server to perform the wrong task.In addition, the use of constants can also improve the maintainability of the Code. If for some reason the value of the constant needs to be changed, we only need to modify one place. For example, in the calculation, at first we take Pi to 3.14, so many of the calculations we use 3.14 to calculate, when the need to increase the accuracy of the calculation, PI needs 3.142, We have to modify all the use of 3.14 code, if the code is more than a long time, not only a large amount of work, but also may be omitted. Feel the benefits of constant! <pre><pre>1 <? PHP 2 Define ("PI", 3.14); 3 $r=3; 4 Echo "area is:". (PI*$r*$r). " "; 5 Echo "perimeter is:". (thePI*$r). " "; 6 ?></pre></pre>Meet System ConstantsPHP provides a large number of predefined constants to any script it runs . however, Many constants are defined by different extensions, and are only present when the extensions are loaded, either dynamically after loading, or they are included at compile Time.There are eight magic constants whose values change as they change position in the Code. For <code>__LINE__</code> example, A value depends on the row it is in the script to Determine. These special constants are case-insensitive, as Follows: <table class="doctable table xhe-border"> <caption> several "magic constants" for PHP </caption> <thead> <tr> <th>name</th> <th> Description</th> </tr> </thead> <tbody class="tbody"> <tr id="constant.line"> <td><code>__LINE__</code></td> <td>The current line number in the File.</td> </tr> <tr id="constant.file"> <td><code>__FILE__</code></td> <td>The full path and file name of the File. If used in the included file, returns the file name that is Included. Since PHP 4.0.2, <code>__FILE__</code> always contains an absolute path (if the symbolic connection is the resolved absolute path), and the previous version sometimes contains a relative path.</td> </tr> <tr id="constant.dir"> <td><code>__DIR__</code></td> <td>The directory where the file Resides. If used in the included file, returns the directory where the included files are Located. It is equivalent to dirname (__file__). Unless it is a root directory, the name in the directory does not include the trailing Slash. (new in PHP 5.3.0) =</td> </tr> <tr id="constant.function"> <td><code>__FUNCTION__</code></td> <td>The name of the function (PHP 4.3.0 new addition). From PHP 5 This constant returns the name (case-sensitive) when the function is Defined. In PHP 4, This value is always in lowercase letters.</td> </tr> <tr id="constant.class"> <td><code>__CLASS__</code></td> <td>The name of the class (PHP 4.3.0 new addition). From PHP 5 This constant returns the name of the class when it is defined (case-sensitive). In PHP 4, This value is always in lowercase letters. The class name includes its declared action area (for example, foo\bar). Note Since PHP 5.4, __class__ has also worked for Trait. When used in the trait method, __class__ is the name of the class that invokes the trait Method.</td> </tr> <tr id="constant.trait"> <td><code>__TRAIT__</code></td> <td>Trait's name (PHP 5.4.0 new plus). From PHP 5.4 This constant returns the name of the trait when it is defined (case-sensitive). The Trait name includes its declared function area (for example, foo\bar).</td> </tr> <tr id="constant.method"> <td><code>__METHOD__</code></td> <td>The method name of the class (PHP 5.0.0 new addition). Returns the name of the method when it is defined (case-sensitive).</td> </tr> <tr id="constant.namespace"> <td><code>__NAMESPACE__</code></td> <td>The name of the current namespace (case-sensitive). This constant is defined at compile time (PHP 5.3.0 is new).</td> </tr> </tbody> </table> how constants are evaluatedConstants are defined, then constants are used, so how do we get constant values?There are two ways to take a value for a constant Value.The first is to get the value directly using the constant name, for example to calculate the area of pi, as Follows:<pre><pre>1 <? PHP 2 Define ("PI", 3.14); 3 $r=1; 4 $area PI*$r*$r// Calculate the area of the Circle 5 ?></pre></pre>The second is the use of the constant () Function. It is the same as the direct use of the output of the constant name, but the function can be dynamically output different constants, in the use of flexible, convenient, The syntax format is as Follows:<pre class="code"><pre class="code">Mixed constant (string Constant_name)</pre></pre>The first parameter, constant_name, is the name of the constant to get, or a variable that stores the constant Name. If successful, the value of the constant is returned, and failure indicates that the error message constant is not defined. (note: mixed indicates that the return value type of the function is of many different types, string indicates that the parameter type is a string Type)How to determine if a constant is definedIf the constants are repeatedly defined, the PHP parser will issue a "Constant XXX already defined" warning to remind us that the constant has been defined. So how do we determine if a constant is defined when team development, or the amount of code is great?The defined () function helps us to determine whether a constant has been defined and its syntax is:<pre class="code"><pre class="code">BOOL defined (string Constants_name)</pre></pre>It only has the parameter constant_name, refers to gets the name of the constant, returns the Boolean type True if present, otherwise returns the Boolean type false; (note: bool means the return value type of the function is a Boolean Type) PHP Reading notes (3)-constants

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