PHP Defining Function Logical -

i starting out learning php, , wondering can me out logical of defining functions. can't seem wrap head around it.

simple example:

<?php  function hello($word) {   echo "hello $word"; }    $name = "john";   strtolower(hello($name))  ?> 

i know if use return instead of echo in function , echo outside defining function, "strtolower" applies, in example doesn't. don't how php interpreting it. in advance.

imagine function box of unknown content. put in , comes out - meaning have parameters , returned.

in code example don't return anything, instead echo string.

function hello($word) { //             ^ parameter   return "hello $word"; }  $name = "john"; strtolower(hello($name)); 

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to explain bit further, can @ original code way:

echo strtolower(hello("john"));      ^          ^--- call hello("john")      |                happens (your echo)      |               hello() ended without return, return null default      |--- call strtolower( null )             happens           strtolower() returned "" 

but want this:

echo strtolower(hello("john"));      ^          ^--- call hello("john")      |                happens      |               hello() return "hello john"      |--- call strtolower("hello john")             happens           strtolower() returned "hello john"