PHP Variables, Concatenation

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In programming, variables are containers that hold values or data. They are used to store and manipulate data in a program.

In most programming languages, variables have a name that identifies them and a data type that specifies the type of data that they can hold. Some common data types include integers (whole numbers), floats (decimal numbers), strings (text), and boolean (true/false values).

Variables can be assigned values using an assignment operator, such as "=". Once a value is assigned to a variable, it can be used and manipulated throughout the program. 

Variables can be used to perform calculations, concatenate strings, or store user input, among other things. They are an important concept in programming and are used extensively in most programming languages.

<?php 

$name = "John";
$last_name = "Smith";
$street = "Main Street";
$house_number = 54;

?>

Four variables are declared and initialized with values:

• $name is initialized with the string value "John"
• $last_name is initialized with the string value "Smith"
• $street is initialized with the string value "Main Street"
• $house_number is initialized with the integer value 54

Variables in PHP are typically declared using the "$" symbol followed by the variable name. The variable name can contain letters, numbers, and underscores, but must start with a letter or underscore. Variable names are case-sensitive in PHP, so $name and $Name would be two different variables.

Once variables are declared and initialized, their values can be used in the program. 

<?php 

$name = "John";
$last_name = "Smith";
$street = "Main Street";
$house_number = 54;

echo "Name: $name" . "<br>";
echo "Last Name: $last_name" . "<br>";

//You can have <br> inside quotes
echo "Street: $street <br>";
echo "House Num: $house_number <br>";

?>

The echo statement is used to output the values of the variables to the browser. The variables are used directly within double quotes. 

Using double quotes with variables allows for variable interpolation, which is the process of including the value of a variable directly within a string. When a string is enclosed in double quotes, PHP will evaluate any variables contained within the string and replace them with their corresponding values.

<?php 

$name = "John";
$last_name = "Smith";
$street = "Main Street";
$house_number = 54;

echo "$name $last_name $street $house_number";
echo "<br>";
echo $name ." ". $last_name ." ". $street ." ". $house_number;

?>

After initializing the variables, the code uses two echo statements to output the values of the variables to the screen.

The first echo statement uses double quotes to interpolate the values of the variables into a single string. The values are separated by spaces, so the resulting string would look like this: "John Smith Main Street 54". The "<br>" tag is used to add a line break after the string.

The second echo statement concatenates the values of the variables with spaces and outputs them as a single string. This time, the concatenation is done using the . operator, which combines the values of the variables into a single string. The resulting string is the same as the first one: "John Smith Main Street 54".

Both echo statements produce the same output, but the second statement explicitly concatenates the values of the variables and might be slightly faster than using string interpolation with double quotes.

Concatenation is the process of combining two or more strings or values into a single string or value. In programming, concatenation is often used to build longer strings by combining smaller strings or adding variables to a string.

In PHP, concatenation is done using the "." operator.