- print_r: documentation ( source)
- preg_match_all: documentation ( source)
<?php
$string = '[vc_row][vc_column][/vc_column][/vc_row][vc_row][vc_column width="1/2"][vc_column_text css=".vc_custom_1576642149231{margin-bottom: 0px !important;}"]This is the string I want to fetch[/vc_column_text][/vc_column][/vc_row]';
preg_match_all("~\[vc_column_text[^][]*\]\s*(.+?)\[/vc_column_text]~", $string, $matches);
print_r($matches[1]);