PHP Variables and Constants
Variables and Constants are different types of identifiers (names) for saving a simple value in PHP. Both are case-sensitive. For the easy identification of Constants and Variables, you should use lowercases for Variables and uppercases for Constants.
1. Variables
They are used more times than constants. As the word already says a Variable is variable and can change its value 100 times and more while the execution of a PHP script. Variables always starts with a dollar sign. Directly after the dollar sign, there are no numbers allowed. But you can use a number after the first letter. Spaces, minus sign and other special chars are also not allowed. To separate a word within a Variable use the underscore sign.
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<?php
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$2myvariable="sweet cat"; // Wrong
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$my2variable="sweet cat"; // Correct
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$myvariable2="sweet cat"; // Correct
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$my-variable="sweet cat"; // Wrong
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$my_variable="sweet cat"; // Correct
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?>
Example on how to use a Variable in PHP:
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<?php
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$my_variable="sweet cat";
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echo $my_variable; // will output: sweet cat
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$my_variable=ucwords($my_variable);
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echo $my_variable; // will output: Sweet Cat
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$my_variable="Sweet Dog";
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echo $my_variable; // will output: Sweet Dog
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$my_variable=strtolower($my_variable);
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echo $my_variable; // will output: sweet dog
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?>
There are 4 times the same Variable name in the same PHP script, but it has 4 different outputs.
2. Constants
A constant should be used for a value which will never be changed in a PHP script. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.
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<?php
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define("MYCONSTANT", "My Constant Value 1"); // Correct
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define("MYCONSTANT2", "My Constant Value 2"); // Correct
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define("2MYCONSTANT", "My Constant Value 3"); // Wrong
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define("MY_CONSTANT", "My Constant Value 4"); // Correct
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define("MY-CONSTANT", "My Constant Value 5"); // Wrong
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?>
As the name already says, that value cannot change during the execution of a php script.
Example on how to use a Constant in PHP:
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<?php
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define("MYBASEURL", "http://www.developerstutorial.com/");
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echo MYBASEURL; // will output: http://www.developerstutorial.com/
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?>
Using a Constant will always output the first defined value.
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<?php
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define("MYBASEURL", "http://www.developerstutorial.com/");
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echo MYBASEURL; // will output: http://www.developerstutorial.com/
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define("MYBASEURL", "http://www.mydomain.com/");
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echo MYBASEURL; // will also output: http://www.developerstutorial.com/
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define("MYBASEURL", str_replace("/","",MYBASEURL));
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echo MYBASEURL; // will also output: http://www.developerstutorial.com/
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?>
So, here are 3 different definitions, but you will always get the output of the first definition.
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