Functions are used to reduce the repetition of codes in your scripts.

The user define function are starts with function tag.

The function name should not start with numbers ( 0 – 9 ). The allowed format is letters ( a-z, A-Z )  and underscores ( _ ).

Beginning with curly braces (  {  ) indicates the function get start to execute the block of code under the function. Closing curly braces ( } ) indicates the closing / end of the function.

<?php
     function cityDetails($city){
         echo "Am staying in $city";
         echo "<br />";
     }

     cityDetails("New York");
     cityDetails("Los Angeles");
?>

Output:
Am staying in New York
Am staying in Los Angeles

<?php
     function add($a,$b){
       $c = $a + $b;
       return $c;
     }
     function sub($a,$b){
       $c = $a - $b;
       return $c;
     }
     function mul($a,$b){
       $c = $a * $b;
       return $c;
     }

    $addingValues = add(25,15);
    $subtractingValues = sub(30,10);
    $multiplicationValues = mul($addingValues,$subtractingValues);
    echo "Multiplication Value is :: $multiplicationValues";
?>

Output :
Multiplication Value is :: 800

// addition value is 40
// subtraction value is 20
// Multiplication of both the data will be 40 * 20 = 800

For more info please refer php.net functions section.