Your string contains a whole mess of / which would need to be escaped as \/ when using / as the regex delimiter. Instead of / as the regex delimiters, use something which won't occur in your string like ~ for example. You must choose a delimiting character which is guaranteed not to appear in $Src, however. You might be safer even with | than with ~.
$Src = 'images/pages/clients/logos/clnt_aljareera_img.jpg';
// Delimit the regular expression with ~
$pttn= '~&Src:'.$Src.'~';
$string=preg_replace($pttn,'',$string,1);
What has happened is your regex delimited by / encounters a p immediately after images/ because it thinks it has reached the closing delimiter. The next word pages is mistakenly treated as a string of regex modifiers.
PHP sees the regular expression:
/&src:images/pages
