According to PHP official documentation, there is no difference between count() or sizeof() function in PHP, which means both are the same, the sizeof() function is just the alias of count function i.e. the sizeof() uses the same as count function underneath. Both are used to count elements in an array.sizeof() function is an alias of count() function used in PHP. count() function is faster and butter than sizeof().

count() method: 

The count() method is used to calculate all the elements in the array or any other countable object. It can be used for both uni-dimensional as well as multi-dimensional arrays. 

count(array)

Example:-

<?php
        $parts = array(

                "Ram" => array(
                "SSD",
                "Processor"
            ),
                "Key" => array(
                "Monitor"  
            ),
                "Mouse" => array(
                "Cebal"
            )
        );
        
        print_r($parts);
        print("<br>");
        
        echo "Sub elements of an array: "
            . count($parts) . "<br>";
        echo "All elements of an array: "
            . count($parts, 1);
        
?>

Output:-

sizeof() method: 

The sizeof() method is used to calculate all the elements present in an array or any other countable object. It can be used for both uni-dimensional as well as multi-dimensional arrays. 

sizeof(arr, mode)

Example:-

<?php
    $parts = array(

        "Computer" => array(
        "SSD",
        "Processor"
    ),
        "Keyboard" => array(
        "Monitor"  
    ),
        "Mouse" => array(
        "CPU"
    )
);
    
    print_r($parts);
    print("<br>");
    
    echo "Sub elements of an array: "
        . sizeof($parts) . "<br>";
    echo "All elements of an array: "
        . sizeof($parts, 1);
 
?>

Output:-

Difference between sizeof() and count() methods:

• The sizeof() method takes a longer execution time.
• The sizeof() method is an alias of the count() method.