how to Removes all characters except digits from a variable in php code example

Example 1: php remove non numeric

$res = preg_replace("/[^0-9]/", "", "Every 6 Months" );

Example 2: php remove characters not numbers or letters

$res = preg_replace("/[^a-zA-Z0-9\s]/", "", $string);

Example 3: print only some characters of a string in php

substr(string,start,length)