$ ag '(\$[0-9a-zA-Z_]+|\))(\[[\w$]+\])?{[^\n=}]+}' --php | sort | uniq

(removed 3rd part libraries and obvious false positives)

backup/cc/cc_lib/gral_lib/pathutils.php:107:        $chr = $path{$count};

backup/cc/cc_lib/gral_lib/pathutils.php:109:            $path{$count} = DIRECTORY_SEPARATOR;

backup/cc/cc_lib/gral_lib/pathutils.php:121:        $chr = $path{$count};

backup/cc/cc_lib/gral_lib/pathutils.php:123:            $path{$count} = '/';

backup/cc/cc_lib/gral_lib/pathutils.php:91:        $chr = $path{$count};

backup/cc/cc_lib/gral_lib/pathutils.php:93:            $path{$count} = '/';

draftfile.php:44:} else if ($relativepath{0} != '/') {

enrol/ldap/lib.php:437:                        $idnumber = $course{$this->config->course_idnumber}[0];

file.php:45:} else if ($relativepath{0} != '/') {

lib/classes/text.php:724:        $ord0 = ord($utf8char{0});

lib/classes/text.php:728:        $ord1 = ord($utf8char{1});

lib/classes/text.php:732:        $ord2 = ord($utf8char{2});

lib/classes/text.php:736:        $ord3 = ord($utf8char{3});

lib/moodlelib.php:5737:    switch ($modargs{0}) {

lib/weblib.php:2981:        if (rand(0, 2) && $email{$i}!='@') { // MDL-20619 some browsers have problems unobfuscating @.

lib/weblib.php:2982:            $obfuscated.='%'.dechex(ord($email{$i}));

lib/weblib.php:2984:            $obfuscated.=$email{$i};

lib/wiki_to_markdown.php:150:      $listchar = $line{0};

mod/quiz/report/statistics/tests/statistics_test.php:160:                if ($start = ($items[$key]{0}=='"')) {

mod/quiz/report/statistics/tests/statistics_test.php:162:                    while (!$end = ($items[$key]{strlen($items[$key])-1}=='"')) {

question/engine/questionusage.php:704:                if ($responsedatakey{0} === '-') {

question/format/gift/format.php:286:        } else if ($answertext{0} == '#') {