Main.php

<?php
/**
 * Sample PHP script to check if a string is available to use in namespace.
 * 
 * View this script in action online:
 *   https://paiza.io/projects/cjheAXgzbvk4uEq8civKAA
 * View this script in Gist:
 *   https://gist.github.com/KEINOS/b72391409f201777f8c4420a59a9c282
 */

namespace SAMPLE;

echo 'PHP Version: ', phpversion(), PHP_EOL, PHP_EOL;

$list_sample = [
    // Available
    'FOO',
    'FOO_',
    '__FOO__',
    'Vendor\Package\Valid',
    'SAMPLE', // Already user defined namespace but not used

    // Not available names
    'KEINOS', // Already user defined and used namespace
    'isavailablestring', // User defined function name
    'stdClass', // Existing class name
    'stdclass', // Existing class name
    'sort', // Internaly defined function name
    'FOO-',
    'FOO-BAR',
    '$this',
    '\KEINOS',
];

foreach($list_sample as $string){
    echo \KEINOS\isAvailableNamespace($string) ? 'OK' : 'NG', ': ', $string, PHP_EOL;
}

/*
 * =============================================================================
 *  Functions
 * =============================================================================
 */

namespace KEINOS;

function isAvailableNamespace(string $string): bool
{
    // Eventhough, class names are case-sensitive, to avoid confusion do NOT allow
    // use same class or function name.
    $name = strtolower($string);
    
    $defined_names = array_flip(getListNamesUsedInLowerCase());
    if(isset($defined_names[$name])){
        return false;
    }

    // Check available characters to use
    //   Ref:
    //     - https://stackoverflow.com/questions/41027816/what-are-the-valid-characters-in-php-namespace-names
    //     - https://www.php.net/manual/en/language.variables.basics.php
    return preg_match('/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff\\\\]*[a-zA-Z0-9_\x7f-\xff]$/', $string);
}

function getListNamesUsedInLowerCase(): array
{
    static $result;
    if(isset($result)){
        return $result;
    }

    $defined_names = [];
    $defined_names += array_flip(get_declared_classes());
    $defined_names += array_flip(get_defined_functions()['internal']);
    $defined_names += array_flip(get_defined_functions()['user']);

    $result = [];
    foreach($defined_names as $name => $value){
        $name = strtolower($name);
        if(strpos($name, '\\') === false){
            $result[] = $name;
            continue;
        }
        // Split names with namespace
        $split = explode('\\', $name);
        $result = array_merge($result, $split);
    }

    return $result;
}