$arr = array();
$arr['key'] = 'value';

$arr = array("a"=>1,"b"=>2,"c"=>3);

<?php

$arr = array("a"=>1,"b"=>2,"c"=>3);

foreach($arr as $key => $val){
        print $key . ":" . $val\r\n;
}

a:1
b:2
c:3

<?php
    $arr = array("aaa"=>1,"bbb"=>2,"ccc"=>3);

    if(array_key_exists("aaa",$arr)){
        //nop
    }else{
        $arr["aaa"] = "val";
    }

    foreach($arr as $key => $val){
        print $key . ":" . $val . "<br />";
    }
?>

aaa:1
bbb:2
ccc:3