it should be noted that an array with one or no elements works fine. for example:

<?php
    $a1 = array("1","2","3");
    $a2 = array("a");
    $a3 = array();
    
    echo "a1 is: '".implode("','",$a1)."'<br>";
    echo "a2 is: '".implode("','",$a2)."'<br>";
    echo "a3 is: '".implode("','",$a3)."'<br>";
?>

will produce:
===========
a1 is: '1','2','3'
a2 is: 'a'
a3 is: ''

It's not obvious from the samples, if/how associative arrays are handled. The "implode" function acts on the array "values", disregarding any keys:

<?php
declare(strict_types=1);

$a = array( 'one','two','three' );
$b = array( '1st' => 'four', 'five', '3rd' => 'six' );

echo implode( ',', $a ),'/', implode( ',', $b );
?>

outputs:
one,two,three/four,five,six

Can also be used for building tags or complex lists, like the following:

<?php

$elements = array('a', 'b', 'c');

echo "<ul><li>" . implode("</li><li>", $elements) . "</li></ul>";

?>

This is just an example, you can create a lot more just finding the right glue! ;)

If you want to implode an array of booleans, you will get a strange result:
<?php
var_dump(implode('',array(true, true, false, false, true)));
?>

Output:
string(3) "111"

TRUE became "1", FALSE became nothing.

It might be worthwhile noting that the array supplied to implode() can contain objects, provided the objects implement the __toString() method.

Example:
<?php

class Foo
{
    protected $title;

    public function __construct($title)
    {
        $this->title = $title;
    }

    public function __toString()
    {
        return $this->title;
    }
}

$array = [
    new Foo('foo'),
    new Foo('bar'),
    new Foo('qux')
];

echo implode('; ', $array);
?>

will output:

foo; bar; qux

If you want to implode an array as key-value pairs, this method comes in handy. 
The third parameter is the symbol to be used between key and value.

<?php
function mapped_implode($glue, $array, $symbol = '=') {
    return implode($glue, array_map(
            function($k, $v) use($symbol) { 
                return $k . $symbol . $v;
            }, 
            array_keys($array), 
            array_values($array)
            )
        );
}

$arr = [
    'x'=> 5, 
    'y'=> 7, 
    'z'=> 99,
    'hello' => 'World',
    7 => 'Foo',
];

echo mapped_implode(', ', $arr, ' is ');

// output: x is 5, y is 7, z is 99, hello is World, 7 is Foo

?>

It may be worth noting that if you accidentally call implode on a string rather than an array, you do NOT get your string back, you get NULL:
<?php
var_dump(implode(':', 'xxxxx'));
?>
returns
NULL

This threw me for a little while.

Even handier if you use the following:

<?php
$id_nums = array(1,6,12,18,24);

$id_nums = implode(", ", $id_nums);
                
$sqlquery = "Select name,email,phone from usertable where user_id IN ($id_nums)";

// $sqlquery becomes "Select name,email,phone from usertable where user_id IN (1,6,12,18,24)"
?>

Be sure to escape/sanitize/use prepared statements if you get the ids from users.

null values are imploded too. You can use array_filter() to sort out null values.

<?php
$ar = array("hello", null, "world");
print(implode(',', $ar)); // hello,,world
print(implode(',', array_filter($ar, function($v){ return $v !== null; }))); // hello,world
?>