Sorting characters by count using PHP or Python

Question

I have a string of characters

abcdefghijklmnopqrstuvwxyz_

I want to take this string of characters and sort them by the number of times they appear in a large block of characters. For example:

cwrxwzbgickpjbp_svnudntddwdqbfgzyiqpuxddmpvyfquosmicfzkjekxzchngpqaksafulateukuwomdrwza_n_ptzktjzcuibnebe_tqessrzqewgkadrkvtyznaupodanwazopg_fijcoojojbsolr_ejesukzc_quochdnmti_lkvrsegyieqlqysuxdvetkqtkhxaiypfdiddztlicjurnllriopdtuuzpryrsepfydyeg_xkr_ruxp_lgqesysidfsygztwrba_ay_gaqqklbrvr_lbhawjraqujfxptmuvqfzklfodgaqrnhjravksjwemoosdlxtvw_qspxmlvqryusfixzlkb_p_c_tepzozzwnokvqspkizygoqpbhjnsxopchzgapctowbrletrunlgnvzpfwrqgedo_s_ygkxz_mpncnve_gfpbotupawevhfxvqhwlerupjfibosbvhiijrodigzyhy_iijes_xsqorshhdzkjqitpljsftpitjetwmzqiabyiewgtbjaddtsjkckcxxvlyrchloetluxkohn_uihkdjpcqgvejanslakmwendgkmvmayknvjjnr_kdapnumwvz__lsimxdtrflyleykxejl_jbkhexpcyreoapelqzzyriyrbxdgbgwrrxlj_pt_mpwubvbveakxfsbfgj___

I also want to drop any characters after and including the underscores once I have these sorted.

Is recursion the right idea I need to look at here?

EDIT

Example of what may be output:

afiskjweocnsdkspwjrhfg

Basically the characters will simply be sorted based on their frequency in a single line.

Answer 1

<?php

$text = 'ahugechunkofatext';
$charCounts = count_chars($text, 1);
arsort($charCounts);

$chars = array_map('chr', array_keys($charCounts));
$chars = array_filter($chars, function ($char) {
    return !in_array($char, ['_']); // A list of chars that you don't want
});

echo implode('', $chars) . PHP_EOL;