He creado una función que a partir de unos caracteres, puede reconocer estos en un texto.

lexer_simple_reconoce_textos.php

<?php

$token_stop_character      = [
    '(' =>  'open_parentesis',
    ')' =>  'close_parentesis',
    '=' =>  'igual',
    '[' =>  'open_corchete_cuadrado',
    ']' =>  'close_corchete_cuadrado',
    ' ' =>  'espacio',
    '*' =>  'multiplicación',
    ',' =>  'coma',
    ';' =>  'punto_y_coma',
];
$multi_character_token  = [
    'let'       => 'creacion_variable',
    'const'     => 'creacion_constante',
    'function'  => 'creacion_funcion',
    '=='        => 'comparación',
    '**'        => 'potenciacion',
];
$tokens = array_merge($token_stop_character, $multi_character_token);

$string = 'let hola = (uno) => [ uno ** 2, 5];';

$recurtion = function($stringFn, $iterator = 0, $previous_character = null, $token_index = 0, array $resulting_array = []) 
    use (&$recurtion, $tokens, $token_stop_character, $multi_character_token)
{
    $string_in_iterator     = $stringFn[$iterator] ?? null;
    $string_to_parse        = isset($previous_character) 
        ? ($previous_character . $string_in_iterator)
        : $string_in_iterator;
    $there_is_a_token       = isset($tokens[$string_to_parse]) || !isset($string_in_iterator) || isset($token_stop_character[$stringFn[$iterator + 1]]);

    $capturing_token = ($there_is_a_token)
        ? array_merge($resulting_array, [$token_index => 
                [
                    'token' => $string_to_parse,
                    'type'  => $tokens[$string_to_parse] ?? 'variable',
                ]
            ])
        : $resulting_array;
        
        
    $new_previous_character     = ($there_is_a_token)
        ? null
        : $string_to_parse;
    $new_token_index   = ($there_is_a_token)
        ? $token_index + 1
        : $token_index;
        


    return !isset($string_in_iterator)
        ? $resulting_array
        : $recurtion($stringFn, $iterator + 1, $new_previous_character, $new_token_index, $capturing_token);
};
var_dump($recurtion($string));