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Constants are similar to variables only in the sense that are used to store values, but otherwise, constants and variables differ in many ways.
To create a constant, it is used the define() function instead of the assignment operator (=):

define("CONSTANT_NAME", 'value', [case-insensitive]);
-The case-insensitive parameter at the end of the definition is optional (a boolean value: true or false), and by default is false, meaning that the defined constant is in fact case-sensitive.
- Constants do not use the initial dollar, their name must start with either a letter or an underscore character, followed by letters, numbers, or underscores.
- By default and convention, a defined constant is uppercase, although this is not required.
- A constant can only be assigned a scalar value (a string or a number). And unlike variables, a constant's value cannot be changed.
- To access the value of a constant, make reference to its name.

The following example creates two constants and attempts to recreate one of them.
define("BROTHERS", 'Victor, Alex');
define("COUSIN", "Vasile", true);                // defined case-insensitive

echo 'My brothers: '. BROTHERS. '<br />';        // My brothers: Victor, Alex

// Try to redefine the BROTHERS constant
define("BROTHERS", 'Relu');                      // Notice: Constant BROTHERS already defined in ...

echo 'My cousin: '. COUSIN. '<br />';            // My cousin: Vasile

// assign the constant's value (the one case-insensitive) to a variable
$cname = Cousin;
echo 'My cousin: '. $cname;                     // My cousin: Vasile

You cannot put the constant within quotation marks:
                echo "My brothers: BROTHERS";     results:   My brothers: BROTHERS

The scope of a defined constant is global, meaning it can be used in any defined function or class that is also part of that code file, including any other included files or functions.

• If you want to check if a constant is defined, you can use the defined("CONSTANT") function. This function returns 1 if "CONSTANT" is defined, otherwise returns false.
define("BROTHERS", 'Victor, Alex');
echo defined("BROTHERS");           // 1
- The parameter with the name of the constant must be added within double quotes.

Daily Test with Code Example

What attribute makes the input text box to not be modified by the user?
checked="checked" readonly="readonly" disabled="disabled"
<input type="text" value="fixed-value" readonly="readonly" name="a_name" />
What CSS property allows you to create rounded corners in your webpage design?
background-size border-size border-radius
.class {
  border:2px solid blue;
What instruction displays a confirmation dialog box to the viewer, who must then click OK or Cancel to proceed?
indexOf() confirm() prompt()
var ques = window.confirm("The result of 0+0 is 0?");
if (ques) alert("Corect");
else alert("Incorrect");
Indicate the PHP function that returns the lowest number of the parameter values.
floor() ceil() min()
$min_nr = min(12, 8, 25, 13);
echo $min_nr;        // 8

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