multi line string replace

cant seem to get it to work say this is the string

"           <tr>
               <td colspan="3" valign="top" class="footerline">"

replace with "hello". problem is, i cant even get other functions to get the position of the string. No problems finding or matching replacing

"           <tr>"

or

"                <td colspan="3" valign="top" class="footerline">"

but together no luck. strstr, stristr etc str_replace. also tried

"           <tr>\n                 <td colspan="3" valign="top" class="footerline">"

also tried doing a nl2br on the compare string and actual string first to see if that'd work. replaced the exact same bits.