PHP function argument refers to a value that is passed into a function when it is called. PHP Arguments supports Call by Value, Call by Reference, Default Argument Values and Variable-length argument.

1. Call by Value:

In Call by Value, the value of a variable is passed directly. This means if the value of a variable within the function is changed, it does not get changed outside of the function. 

Example:

<?php  
function increment($i)  
{  
    $i++;  
}  
$i = 10;  
increment($i);  
echo $i;  
?>  

Output:

10

2. Call by Reference:

In PHP, call by reference is a mechanism in which a function can directly modify the original value of a variable outside of the function. By default, function arguments are passed by value in PHP, which means that a copy of the argument’s value is passed to the function. However, you can pass arguments by reference using the & symbol to allow the function to modify the original value of the variable.

Example:

<?php  
function increment(&$i)  
{  
    $i++;  
}  
$i = 10;  
increment($i);  
echo $i;  
?> 

Output:

11

3. Default Argument Values:

When you calling PHP function if you don’t specify any argument, it will take the default argument.

Example:

<?php  
function Hello($name="Amit"){  
echo "Hello $name <br>";  
}  
Hello("Abishek");  
Hello();//passing no value  
Hello("Vijay");  
?>  

Output:

Hello Amit
Hello Abhishek
Hello Vijay

4. Variable Length Argument:

It is used when we need to pass n number of arguments in a function. To use this, we need to write three dots inside the parenthesis before the argument. 

Example:

<?php  
function add(...$nums) {  
    $sum = 0;  
    foreach ($nums as $n) {  
        $sum += $n;  
    }  
    return $sum;  
} 
echo add(2, 4, 6, 8);  
?>  

Output:

20