Get the first letter of each word in a string

You can use the preg_match_all function in PHP to match all the words in a string and then use the substr function to get the first letter of each word. Here is an example:

<?php

$string = "This is a test string";
preg_match_all('/\b\w/', $string, $matches);
echo $firstLetters = implode('', $matches[0]);

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The preg_match_all function searches for all occurrences of the regular expression /\b\w/ in the string, which matches the first letter of each word. The implode function is used to join the array of first letters into a single string.